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

Twenty 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. Each has a hand-drawn cross-section or plan-view schematic; the synoptic-scale systems also carry a real 850-hPa temperature (shaded) + 500-hPa height (contours) map for the case date, from ERA5. Forecaster cases are quoted from NWS Area Forecast Discussions with a link to the source text.

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: 10-m wind + T2m + dewpoint; radar clear-air fine-line. (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

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; 850-hPa T; radar fine-line.

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

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: surface T + wind convergence; radar. (Mesoscale.)

Nov–Mar · OKX / BOXthe classic New England / Long Island rain–snow line.

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 known MPAS warm-bias — models burn the stratus off too fast.

Diagnose with: GOES visible/IR loop, ceilometer.

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)

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.

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

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.

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

ERA5 · 2007-04-02 12Z

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 — models erode the wedge too fast (warm bias).

Diagnose with: surface θ / mesonet, 850-hPa T, the in-situ pressure ridge.

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

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 — a mechanism MPAS handles well (resolved terrain).

Diagnose with: NW gradient wind, falling dewpoint, 850-hPa thermal ridge.

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

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: radar bands, 850-hPa T minus lake temp, boundary-layer depth, fetch.

Nov–Jan · lee of the Great Lakessingle bands can exceed 5 cm/hr with thundersnow.

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 — MPAS runs too warm (stable-PBL scheme under-cools).

Diagnose with: sky cover + wind + dewpoint, snow analysis, sfc cooling rate.

Cold season · interior valleysthe min can be 15–20 °F colder in a snow-covered valley than at the airport.

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: 12Z sounding cap strength, profiler, surface T trend.

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 T/θ advection, low-level jet, overnight trend. MPAS handles this well.

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

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: VAD wind profile, 925/850-hPa wind max, decoupling time.

Warm season · overnightstrongest over the Plains; its moisture plume reaches the NE ahead of warm fronts.

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: the vertical T sounding (twice crossing 0 °C), surface wet-bulb, CAD strength.

DJF · LWX / PHI / OKXthe Piedmont / interior-NE ice-storm corridor east of the mountains.

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.

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

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: 500-hPa shortwave speed, thickness fall, post-frontal wind.

DJF · northern tier → NElittle snow, big cold — the drop behind it is the story.

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.

Warm season · afternoon/eveninga passing outflow can knock 10–15 °F off in minutes, then recover.

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.

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

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.

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.

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)

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.

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).