Special Weather Systems of the U.S. & the Northeast

Twenty-one recurring circulations — from the Boston sea breeze to the Appalachian cold-air wedge — read as mechanisms: what forces them, what they do to the near-surface temperature, and where they lead. Every system carries both a hand-drawn cross-section or plan-view schematic and a real ERA5 map of its case date — 850-hPa temperature (shaded) + 500-hPa geopotential height (contours). For the storm- and mesoscale systems the ERA5 map shows the synoptic setup that permits them; the feature itself lives below reanalysis resolution (look to radar). Forecaster cases are quoted from NWS Area Forecast Discussions with a link to the source text.

What actually diagnoses these systems — and why the maps above are the wrong instrument. Every card's "Diagnose with" line below reports what the peer-reviewed literature says, not what convention assumes. Reading 53 papers produced exactly one result that generalises, and it is a surprise: for every system on this page where a primary source could be verified, the controlling quantity is a layer — a depth, a difference between two levels, an integral through the whole profile — and it cannot be recovered from any single pressure level. Lake-effect snow needs the lake temperature minus the 850-hPa temperature, not either one alone. Freezing rain needs the melting and refreezing energy integrated down the sounding. A capping inversion needs a lapse rate sustained over 200 hPa. An Alberta clipper needs the offset between the 500-hPa vorticity maximum and the surface low. Cold-air damming shows no expression at all at 850 hPa — it lives near 930 hPa, underneath the level everyone reaches for. Five independent papers arrive here by five different physics. So the 850-hPa temperature and 500-hPa height maps on these cards are context: they show the synoptic setup that permits each system, and they are not the diagnostic for any of them. 🔶 The clause "single levels are insufficient" is our own synthesis — none of those five papers says it in so many words, and that gap is exactly what the tags below are for. ✅ the paper says it in words and the quote is printed here · 🔶 it follows from the paper's methods but the paper does not state it — never a quotation · ⚠️ a wording limit or a contrary nuance we are not smoothing over · ❌ we searched, twice, and nothing survived. An honest gap, printed rather than filled with a plausible guess.

ERA5 · 2007-03-27 18Z (synoptic setup)

Schematic — NOAA/NWS JetStream — Sea-breeze circulation. Public domain.

Mesoscale · thermally direct

Sea breeze

A daytime onshore circulation set up by the land heating faster than the sea.

Cause

Differential land–sea heating drives onshore flow beneath a return flow aloft.

Affects

The advancing sea-breeze front shifts wind onshore, raises dewpoint, and caps the coastal high by late morning.

Leads to

A sharp coast-vs-inland split; the convergence line can trigger inland showers.

Diagnose with: the thermodynamic front and the kinematic front separately — the sea-breeze front bifurcates, and the two are not co-located, so tracking 10-m convergence alone mis-locates the T2m drop by up to ~15 km: the convergence line and the temperature drop are different objects. 🔶 Inferred from Miller, Keim, Talbot & Mao 2003, Rev. Geophys. (after Atkins et al. 1995) — no DOI recorded in our review; the paper does not phrase it this way. Then: 10-m wind shift, dewpoint gradient, radar clear-air fine-line. ✅ Timing: the coastal daily maximum runs ~1–2 h earlier on sea-breeze days — 1800–1900 UTC versus ~2000 UTC on fair-weather days, with Wantagh "cool[ing] almost 2 h earlier (1900 UTC)" — McCabe & Freedman 2023, Wea. Forecasting 38(4), 10.1175/WAF-D-22-0119.1. (Mesoscale — below reanalysis resolution.)

2007-03-27 · Boston (BOX) — source"eastern coastal MASS where temps have cooled into the mid 40s courtesy of the seabreeze front."

ERA5 · 2008-03-08 18Z

Schematic — NASA (SciJinks) via Wikimedia Commons — Backdoor cold front. Public domain.

Synoptic · shallow front

Backdoor cold front

A cold front that arrives from the northeast — backwards from the usual NW→SE.

Cause

A 500-hPa ridge over eastern Canada and a surface high over Québec/the Maritimes drive cool marine air southwest down the coast.

Affects

A coastal crash of 10–30 °F behind an NE wind shift and dewpoint drop; the interior often stays hot.

Leads to

Big coast-vs-inland splits and forecast busts — a spring/early-summer specialty.

Diagnose with: MSLP + 10-m wind + dewpoint; the NE wind shift; radar fine-line. ❌ Forecaster practice, not a cited result. The backdoor front is not covered anywhere in our 53-paper review — it was neither verified nor formally searched, so nothing on this line is carried by a source. Note in particular that the 850-hPa temperature is dropped from this line: this is a shallow front, and no verified result supports reading it at 850 hPa.

2008-03-08 · Boston (BOX) — source"the backdoor cold front that cooled temperatures into the upper 30s and lower 40s."

ERA5 · 2008-01-26 12Z

Mesoscale · baroclinic zone

Coastal front

A near-stationary temperature boundary hugging the coastline in the cool season.

Cause

Land–sea thermal contrast (often reinforced by cold-air damming) concentrates a shallow baroclinic zone at the coast.

Affects

A very tight T and precip-type gradient — a few km separate 34 °F rain/snow from 45 °F.

Leads to

Rain/snow lines and a favored axis for coastal cyclogenesis.

Diagnose with: Petterssen frontogenesis computed from the total observed 10-m wind — the convergence term, explicitly not the geostrophic/deformation term; SST and air–sea ΔT (the SST is the necessary condition); 700-hPa height and wind — this, not 500, is the steering level; and 850-hPa thermal advection, i.e. t850 together with the 850-hPa wind — the t850 value on its own is useless here. ✅ Quoted: "the initial coastal development takes place in the absence of a geopotential perturbation at 500 mb" … "The steering level for the zipper low appears to be 700 mb." … "geostrophic deformation was incapable of initiating coastal frontogenesis." — Keshishian & Bosart 1987, Mon. Wea. Rev. 115, 100–117 (no DOI printed in the PDF — 1987 AMS typesetting predates DOI assignment). That last line is the one that matters: a height-derived, geostrophic frontogenesis diagnostic does not merely under-detect this front — it carries the wrong sign, degenerating into a cellular four-quadrant pattern where the observed wind gives a clean coast-parallel axis. Baroclinicity is "concentrated below 800 mb"; the cold layer is ~300 m deep. See also Bosart 1975, QJRMS 101, 957–978, and Nielsen 1989, Mon. Wea. Rev. 117, 1380–1401 (neither prints a DOI). (Mesoscale.)

2008-01-26 · Boston (BOX) — source"caveat: coastal front developing /ESE wind/… snow showers may break out Boston to NBT."

ERA5 · 2009-05-19 15Z

Schematic — NOAA/NWS JetStream — Land–sea temperature contrast. Public domain.

Boundary layer · low cloud

Marine layer & sea fog

Cool moist marine air trapped under an inversion, filled with stratus and fog.

Cause

Warm moist air over cold shelf water; a subsidence inversion caps a shallow saturated layer.

Affects

Suppressed insolation and a cool, late-peaking coastal high; dense fog.

Leads to

A coastal warm bias when the stratus clears sooner than forecast. ❌ Which model, and by how much, is not something we can source: our 53-paper review found no paper evaluating MPAS 2-m temperature bias at convection-permitting resolution, and the marine layer itself returned no primary source in two searches.

Diagnose with: GOES visible/IR loop, ceilometer; air–sea ΔT; cloud fraction and cloud base. ❌ Forecaster practice, not a cited result. The marine layer / coastal stratus / sea fog is one of the systems our 53-paper review searched twice and found no primary source for. The burn-off claim in the panel above should be read the same way: the review also found no paper at all evaluating MPAS 2-m temperature bias at convection-permitting resolution, so no citation supports it either.

2009-05-19 · Boston (BOX) — source"cooler marine-influenced region… Logan may have its warmest temps around 6 PM?"

ERA5 · 2015-01-27 00Z (Jan 2015 blizzard)

Schematic — NOAA/NESDIS — Nor'easter formation schematic. Public domain.

Synoptic · cyclogenesis

Nor'easter

An intense coastal cyclone with a long fetch of northeasterly winds off the Atlantic.

Cause

Baroclinic cyclogenesis along the coast / Gulf-Stream gradient, forced by an upper trough and jet.

Affects

Heavy snow/rain NW of the track, damaging NE winds, coastal surge.

Leads to

The Northeast's signature high-impact winter storms.

Diagnose with: MSLP + 500-hPa height/vorticity; IR comma cloud; radar bands. ❌ Forecaster practice, not a cited result. The nor'easter — and the Miller-A-versus-B distinction — is one of the systems our 53-paper review searched twice and found no primary source for. The 500-hPa field here is convention, not a verified diagnostic; it is on this line because forecasters use it, not because a paper says it works.

27 Jan 2015 · OKX / BOXa benchmark coastal low; 60–90 cm snowfall across eastern New England.

ERA5 · 2011-01-27 00Z

Schematic — NOAA/NWS State College (CTP) — Miller Type B redevelopment. Public domain.

Synoptic · redevelopment

Miller-B redevelopment

A primary low fills over the Appalachians while a new low jumps to the coast.

Cause

The inland low weakens crossing the mountains; upper energy transfers to a secondary coastal low.

Affects

A sudden coastal intensification and a jump in the snow gradient.

Leads to

Notorious busts — the storm "reloads" on the coast hours later.

Diagnose with: MSLP tendency, 500-hPa vorticity handoff, the benchmark 40 °N/70 °W. ❌ Forecaster practice, not a cited result. The Miller-A-versus-B distinction is one of the systems our 53-paper review searched twice and found no primary source for. Every term on this line is forecaster convention.

DJF · OKX / PHIthe classic "Miller B" secondary cyclogenesis.

ERA5 · 2007-04-02 12Z

Schematic — NOAA/NWS Baltimore–Washington (LWX) — CAD cross-section along the Appalachians. Public domain.

Meso-α · shallow cold pool

Cold-air damming (the wedge)

A shallow dome of cold air trapped against the east slope of the Appalachians.

Cause

High pressure to the N/NE forces cold, dense air against the mountains; warm air overruns it aloft.

Affects

Overcast, drizzle, and a strongly suppressed daytime high from DC through NYC (8–20 °F cold).

Leads to

Freezing rain, and busts when the wedge erodes sooner than forecast. ⚠️ Ellis, Marston & Nelson 2018 identify the hard part as the cloud- and precipitation-driven diabatic cooling that sustains the dome — not the dry dynamics. We make no claim about how fast any particular model erodes it: no paper in our review measures that.

Diagnose with — and note what is not here: ① the "U"-shaped ridge in the SLP field (equivalently a U-shaped trough in the sea-level thermal field); ② 930-hPa height and temperature — the base of the capping inversion, and 930 hPa is not a standard pressure level; 925 hPa is the nearest available one; ③ surface T and surface moisture, on both sides of the barrier; ④ 925-hPa wind direction — easterly east of the ridge versus southerly west of it; ⑤ ceiling / cloud base; ⑥ the PNA index 2–3 days earlier, which carries real predictive skill (p = 0.02 at −2 d, p < 0.01 at −3 d). 850-hPa temperature has been removed from this line. ✅ Quoted: "Composite air temperatures depict cold air along the eastern side of the Appalachians at the surface, but no expression of CAD at the 850 hPa level. This coincides with a composite wind field comprised of easterly and northeasterly flow at the surface east of the Appalachians, yet southerly and southwesterly 850 hPa winds across all of the region." — Ellis, Marston & Nelson 2018, Int. J. Climatol. 38, 530–542, 10.1002/joc.5189. And: "the cold dome can be identified by a 'U' shaped ridge (trough) in the sea level isobar (thermal) patterns and the 930-mb height (temperature) fields representative of conditions at the base of the inversion overlying the cold dome." — Bell & Bosart 1988, Mon. Wea. Rev. 116, 137–161 (no DOI printed in the PDF; the legacy AMS DOI string appears in filenames only and is not asserted as read). The dome is almost always confined below 850 hPa; z500 is a broad-scale precursor only — never an identifier. ⚠️ Ellis et al. document a suppressed diurnal amplitude inside the wedge; they make no claim about a shift in the timing of the daily maximum, and neither do we. See also Bailey et al. 2003, Wea. Forecasting 18(4) (objective climatology, SLP-Laplacian detection; no DOI recorded in our review); and, for the Japanese (Kanto) analogue of the same wedge, Suzuki et al., J. Meteor. Soc. Japan 99(1), 10.2151/jmsj.2021-002, where model terrain height — not the turbulence scheme — is the dominant error term.

2007-04-02 · Boston (BOX) — source"stuck in the 40s as a strong wedge of high pressure knifes in from the NNE."

ERA5 · 2008-06-08 18Z

Schematic — Schroeder & Buck 1970 (USDA/USFS), NWCG PMS 425-1 — Downslope / lee warming. Public domain.

Mesoscale · adiabatic

Downslope / lee warming

Air descending the lee of the mountains warms and dries by compression.

Cause

W/NW flow crosses the Appalachians/Berkshires and sinks on the lee side, warming dry-adiabatically.

Affects

A higher, drier maximum at NYC/PHL/DCA and lower dewpoints.

Leads to

Elevated highs and fire-weather days. ❌ The page previously called this "a mechanism MPAS handles well". That was unsourced and has been removed: downslope / lee warming returned no primary source in two searches, and no paper in our review evaluates MPAS 2-m temperature at all.

Diagnose with: NW gradient wind, falling dewpoint, 850-hPa thermal ridge, the lee-slope descent path. ❌ Forecaster practice, not a cited result. Downslope / lee warming is one of the systems our 53-paper review searched twice and found no primary source for. The "MPAS handles this well" claim in the panel above is unverified on the same grounds: the review found no paper evaluating MPAS 2-m temperature bias at all.

2008-06-08 · Boston (BOX) — source"highs nudged upward a few degrees… W downsloping flow expected… E-coast beaches will be hotter."

ERA5 · 2014-11-07 12Z

Schematic — NOAA/NESDIS — Lake-effect snow cross-section. Public domain.

Convective · lake-modified

Lake-effect snow

Cold air crossing a warm lake destabilizes and dumps narrow, intense snow bands downwind.

Cause

Large lake-minus-850-hPa T difference (≥13 °C) plus long fetch drives shallow convection over open water.

Affects

Extremely localized heavy snow and sharp gradients on the downwind shore.

Leads to

Feet of snow over a few km (Tug Hill, Buffalo).

Diagnose with: lake-surface water temperature MINUS 850-hPa temperature — the ~13 °C threshold — plus fetch, boundary-layer depth, and the radar bands. 🔶 t850 undifferenced is meaningless here: the operative variable is a difference, so the lake/water surface temperature carries exactly as much of the diagnosis as t850 does. Niziol 1987, Operational Forecasting of Lake Effect Snowfall in Western and Central New York, Wea. Forecasting 2(4), 310–321 (no DOI recorded in our review). ⚠️ Niziol does use 500-mb height — at synoptic scale, for pattern recognition, not for the lake-effect diagnosis itself. That distinction is the whole point of this page.

2014-11-07 · Boston (BOX) — source"a swath of rain/snow squalls over western NY state… some of this activity is lake-effect enhanced with CAA coming across."

ERA5 · 2016-01-05 06Z

Schematic — Schroeder & Buck 1970 (USDA/USFS), NWCG PMS 425-1 — Cold-air drainage & pooling. Public domain.

Boundary layer · nocturnal

Radiational cooling & cold pools

On clear, calm, dry nights the surface radiates away its heat and cold air drains downhill.

Cause

Strong longwave loss builds a surface inversion; the layer decouples and cold air pools in valleys (aided by snow).

Affects

Drives the daily minimum, frost, and fog; large diurnal range.

Leads to

Valley cold pools, and a nocturnal warm bias when the stable layer is under-resolved. ⚠️ The evidence here is real but it is about WRF and the HRRR, not MPAS — and Suzuki et al. 2021 find model terrain height, not the turbulence scheme, to be the dominant error term. The corpus also disagrees with itself on the scheme: one paper has MYNN under-mixing in a stable cold pool, another has it over-mixing in a stable valley. We are not smoothing that over.

Diagnose with — and not with any mid-tropospheric field: ① cloud fraction ② liquid water path ③ downwelling shortwave and longwave at the surface ④ PBL height ⑤ 10-m wind ⑥ snow cover. ✅ These, not z500 or t850, drive the T2m error in cold pools — Adler, Bianco, Djalalova, Olson & Turner 2023, Geosci. Model Dev. 16, 597–619, 10.5194/gmd-16-597-2023. The numbers to calibrate against: GFSv15, conditioned on ≤50 % sky cover, runs +1.0 °C warm at the 07 h climatological minimum and −1.9 °C cold at the 15 h maximum; conditioned further on ≤5 kt winds the morning warm bias worsens to +1.7 °C. HRRR (MYNN-EDMF) is much flatter: +0.1 → +0.6 °C. Convection-allowing WRF morning boundary layers are too cool and too dry across all five schemes tested — despite near-unbiased PBL depth; depth being right does not mean the profile is (Coniglio, Correia, Marsh & Kong 2013, Wea. Forecasting 28(3), 842–862, 10.1175/WAF-D-12-00103.1). ⚠️ A disagreement we are preserving rather than averaging away: Ntoumos 2023 finds the diurnal range overstated (warm TX + cold TN), while Patel et al. 2021 finds the clear-sky range compressed by ≈3 °C — opposite signs (Patel: GRL, 10.1029/2021GL095101; no DOI recorded in our review for Ntoumos). Also Hu, Nielsen-Gammon & Zhang 2010, J. Appl. Meteor. Climatol. 49(9), 1831–1844, 10.1175/2010JAMC2432.1; and Ma et al. 2024, Meteorol. Appl., 10.1002/met.70020 — where local/TKE schemes best reproduce nocturnal PBL height, yet no scheme reproduced the observed cold-pool depth.

2016-01-05 · Boston (BOX) — source"high pressure overhead along with clear skies/light winds was leading to an ideal night of radiational cooling."

ERA5 · 2007-07-24 18Z

Boundary layer · thermodynamic

Capping inversion & "mixing out"

A warm, dry layer aloft caps the boundary layer until the mixed layer erodes it.

Cause

An elevated mixed layer (warm dry air aloft) lids surface heating; the surface is "stuck" until the mixed layer eats through.

Affects

Temperature flat for hours, then a sudden step-up when it mixes out — or a capped day that never warms.

Leads to

Delayed convective initiation or a warm/cool bust on cap-break timing.

Diagnose with: a lapse rate ≥8 °C km⁻¹ sustained over ≥200 hPa, plus MUCAPE and CIN — every one of them a layer integral, and not one of them readable off a single isobaric level. ✅ Andrews et al. 2024, J. Climate 37(5), 10.1175/JCLI-D-23-0517.1. The 12Z sounding and the profiler are the practical instruments precisely because they are what hand you the layer; the surface T trend then tells you when it broke.

2007-07-24 · Boston (BOX) — source"readings will be held in the upper 70s… decent cap in place and with a lack of surface convergence."

ERA5 · 2008-08-04 12Z

Synoptic · warm advection

Overrunning & the midnight high

Warm air gliding up over a cold surface can make the daily high occur at midnight.

Cause

Ahead of a warm front, low-level warm advection and isentropic upglide raise the temperature through the night.

Affects

The temperature rises after dark; at NYC/BOS ~20 % of winter daily highs occur near midnight.

Leads to

Timing busts — the reason a same-day run must extend past local midnight.

Diagnose with: 850-hPa thermal advection read together with the 850-hPa wind — the t850 value alone carries no advection — plus the low-level jet and the overnight temperature trend. ❌ Forecaster practice, not a cited result — and the sharpest gap on this page. Not one of the 53 papers we reviewed addresses the timing of the daily maximum temperature. What the corpus does find is that models get the phase right and the amplitude wrong: Hu — "the schemes agree on phase, disagree on amplitude"; Cohen and García-Díez — all schemes place the maximum at the observed hour. The only quantified shift anywhere in the corpus is the sea breeze's ~1–2 h earlier maximum (see the sea-breeze card). The "~20 % of winter highs near midnight" figure in the panel above is our own station climatology, not a published result — no paper we found supports a midnight maximum, and citing one for it would be fabrication. The literature never asked this question.

2008-08-04 · Boston (BOX) — source"a warm front approaches Tuesday night… isentropic lift…"

ERA5 · 2008-06-22 06Z

Boundary layer · inertial

Nocturnal low-level jet

After sunset the surface decouples and a fast wind ribbon accelerates just above it.

Cause

Frictional decoupling lets the low-level wind undergo an inertial oscillation and accelerate into a jet.

Affects

Rapid overnight moisture and warm-air transport; feeds dew, fog, elevated convection.

Leads to

Overnight MCS development and the midnight warm-up.

Diagnose with: the U/V wind profile through the jet, the geostrophic wind G, the turbulent stress divergence, and the eddy diffusivity — the VAD profile and the decoupling time are the observational route to all four. 🔶 A full-text search of the primary paper returns zero hits for "500 hPa" and zero for "850 hPa": Van de Wiel et al. 2010, J. Atmos. Sci. 67(8), 10.1175/2010JAS3289.1. ✅ For objective low-level-jet identification (jointly with the sea breeze) in the New York Bight specifically, see McCabe & Freedman 2023, Wea. Forecasting 38(4), 10.1175/WAF-D-22-0119.1.

2008-06-22 · New York (OKX) — source"training of storms possible as the low level jet aligns with upper flow."

ERA5 · 2008-01-03 12Z

Winter · thermal profile

Freezing rain & ice storms

Snow melts in a warm layer aloft, then the rain refreezes on a sub-freezing surface.

Cause

A shallow surface cold layer (often CAD) beneath an elevated warm nose > 0 °C; precip melts then supercools.

Affects

Rain that freezes on contact — ice accretion on every surface.

Leads to

Damaging ice storms; the depth of the cold layer is the whole forecast.

Diagnose with: positive melting energy and negative refreezing energy — depth-weighted integrals of the vertical temperature profile, not any single level's value — plus warm-nose depth, the surface wet-bulb, and CAD strength. 🔶 This is the clearest case on the page in which standard pressure levels are structurally inadequate: the integral needs the profile, and the profile is what pressure levels throw away. The sounding crossing 0 °C twice is the picture; the two energies are the forecast. Bourgouin 2000, Wea. Forecasting (no DOI recorded in our review); Birk et al. 2021, Wea. Forecasting 36(2), 10.1175/WAF-D-20-0118.1; Lackmann, Keeter, Lee & Ek 2002, Model Representation of Freezing and Melting Precipitation: Implications for Winter Weather Forecasting, Wea. Forecasting 17, 1016–1033 (no DOI printed in the PDF).

2008-01-03 · Boston (BOX) — source"southern NH where the deep snowpack may provide sufficient low level cold air for the risk of a period of freezing rain/drizzle."

ERA5 · 2018-01-04 12Z (Jan 2018 bomb)

Synoptic · bombogenesis

Bomb cyclone

A cyclone whose central pressure falls at least 24 hPa in 24 hours.

Cause

Strong baroclinicity, a favorable upper jet/trough, and latent-heat release (over the Gulf Stream) drive explosive deepening.

Affects

Extreme wind, heavy snow, rapid pressure falls over the NE waters and coast.

Leads to

The most intense coastal storms; blizzards and coastal flooding.

Diagnose with: 24-h MSLP tendency, 300-hPa jet, 500-hPa vorticity, SST gradient. ❌ Forecaster practice, not a cited result. Explosive cyclogenesis is not covered anywhere in our 53-paper review — it was neither verified nor formally searched — so nothing on this line is carried by a source. The 24 hPa / 24 h figure in the panel above is the standard definition, not a finding of ours.

4 Jan 2018 · offshore benchmarkpressure fell > 50 hPa; hurricane-force winds off New England.

ERA5 · 2008-01-19 12Z

Schematic — NOAA/NWS State College (CTP) — Alberta clipper typical track. Public domain.

Synoptic · fast mover

Alberta clipper

A fast, moisture-starved shortwave low diving southeast out of Canada.

Cause

A quick Canadian shortwave with little Gulf moisture races SE across the northern tier.

Affects

A swath of light, fluffy snow followed by a sharp shot of arctic cold and gusty NW winds.

Leads to

Temperature crashes and wind chill behind the system.

Diagnose with: the 500-hPa vorticity maximum relative to the SLP minimum — a phase offset between two levels, which is a vertical-structure quantity and not a single-level field — plus the thickness fall and the post-frontal wind. 🔶 The vorticity maximum sits always west of the surface low, so the 500-hPa field alone misplaces the surface low: the shortwave speed you read off z500 is not where the cold arrives. Thomas & Martin 2007, Wea. Forecasting, 10.1175/WAF982.1.

2008-01-19 · Boston (BOX) — source"a clipper moves across the region Tue… suspect might be a little too optimistic with the high temps."

ERA5 · 2013-07-08 18Z

Storm-scale · outflow

Gust front / cold pool

A thunderstorm's rain-cooled downdraft spreads out as a cold, gusty density current.

Cause

Evaporative cooling in the downdraft builds a cold pool that surges outward.

Affects

An abrupt wind shift, gust, and temperature drop at the leading gust front.

Leads to

New convection along the convergence, and micro-timing of T2m a coarse model smears.

Diagnose with: radar thin-line, pressure jump & temperature drop, mesonet. ❌ Forecaster practice, not a cited result. The gust front / thunderstorm cold pool is one of the systems our 53-paper review searched twice and found no primary source for.

2013-07-08 · Boston (BOX) — source"surface analysis shows a weak boundary… perhaps outflow from morning convection is moving through."

ERA5 · 2012-06-29 18Z (heat-ridge "ring of fire")

Meso-β · progressive MCS

Derecho

A long-lived, fast bow-echo complex that produces a swath of straight-line wind damage.

Cause

An organized MCS with a strong cold pool and a descending rear-inflow jet accelerates as a bow echo, often on the north flank of a heat ridge.

Affects

Widespread 60–100 mph winds along a hundreds-of-km track.

Leads to

Major wind damage and outages.

Diagnose with: radar bow & rear-inflow notch, MUCAPE + deep shear, the 850-hPa ring-of-fire. ❌ Forecaster practice, not a cited result. The derecho is one of the systems our 53-paper review searched twice and found no primary source for.

29 Jun 2012 · DC/Mid-Atlantic (LWX)a derecho ran from Indiana to the coast with 80+ mph gusts.

ERA5 · 2013-05-20 21Z (Moore, OK)

Mesoscale · moisture boundary

Dryline

A sharp moisture gradient in the southern Plains that focuses severe convection.

Cause

Moist Gulf air meets dry, well-mixed desert air; the boundary mixes east by day, retreats at night.

Affects

Big dewpoint contrast and convergence across a few km.

Leads to

Supercells and tornado outbreaks (a Plains feature — the NE analogue is the sea-breeze / backdoor Td boundary).

Diagnose with: surface dewpoint gradient, 2-m mixing ratio, convergence, cap. ❌ Forecaster practice, not a cited result. The dryline is one of the systems our 53-paper review searched twice and found no primary source for.

Spring · Southern Plainsthe classic severe-weather focus.

ERA5 · 2012-07-06 12Z (Jul 2012 heat)

Planetary · blocking

Omega block

A persistent, amplified ridge flanked by two lows — an Ω in the 500-hPa flow.

Cause

The jet buckles into a stationary high-amplitude ridge with cutoff lows on each side; it locks in for days.

Affects

Prolonged heat and drought under the ridge, cool and wet on the flanks.

Leads to

Heat waves and air-stagnation; multi-day forecast persistence.

Diagnose with: 500-hPa height anomaly, blocking indices, ridge-axis position. ❌ Forecaster practice, not a cited result. The omega block / blocking-ridge heat wave is one of the systems our 53-paper review searched twice and found no primary source for. This is the one card on the page where a 500-hPa field is the natural instrument — the block is a z500 object — and even here it is convention rather than a verified result.

Jul 2012 · Northeast heatwith the ridge overhead, the daily max ran several degrees above normal for a week.

ERA5 · 2007-10-22 12Z (Oct 2007 CA fires)

Schematic — Schroeder & Buck 1970 (USDA/USFS), NWCG PMS 425-1 — Western downslope winds. Public domain.

Synoptic · offshore downslope

Santa Ana (offshore) wind

High pressure inland drives dry air offshore, warming and drying as it descends to the coast.

Cause

A Great-Basin high forces a gradient toward the coast; air sinks down the mountains, compressing and drying.

Affects

Hot, bone-dry, gusty conditions at the CA coast — the opposite of the marine-layer regime.

Leads to

Critical fire weather. The West-Coast contrast to the NE onshore/sea-breeze world.

Diagnose with: offshore MSLP gradient, 850-hPa warm/dry advection, RH & wind. ❌ Forecaster practice, not a cited result. The Santa Ana is not named anywhere in our 53-paper review; the closest entry is downslope / lee warming, which was searched twice and yielded no primary source either. Nothing on this line is carried by a citation.

Oct 2007 · Southern Californiathe mirror image of the marine layer — downslope wins, and LAX bakes.

Sources. Case maps: ERA5 reanalysis (1.5°, local archive), 850-hPa temperature and 500-hPa height at the stated time. Forecaster quotes: NWS Area Forecast Discussions (WFOs Boston/BOX, New York/OKX, Mount Holly/PHI, Baltimore–Washington/LWX) via the Iowa Environmental Mesonet text archive — each dated case links to its source product. Schematics are hand-drawn (SVG).