Dense fog developed early in the morning, with widespread visibility of 1/4 mile or below. Fog dissipated towards noon.
Every event with a place to put it; 2 filed by county or forecast zone are drawn at its center, in a lighter ring. Tap one for its entry below.
Deaths and injuries are direct and indirect together, as the offices filed them. Damage is property and crops in the dollars of the year, not adjusted for inflation, and only where an office estimated it.
The episode as NWS LWX wrote it up.
Dense fog developed early in the morning, with widespread visibility of 1/4 mile or below. Fog dissipated towards noon.
The local office's own words before the first event, as it sent them, from the Iowa Environmental Mesonet's archive of National Weather Service text.
NWS Baltimore/Washington wrote its last area forecast discussion before the first event at Dec 12, 6:51 AM EST, 9 minutes earlier.
Area Forecast Discussion National Weather Service Baltimore MD/Washington DC 651 AM EST Sat Dec 12 2020 .SYNOPSIS... A low pressure system will pass through the Great Lakes region today, and the trailing cold front will move across the area Sunday. Another low will pass south of the area Monday. High pressure will build to the north Tuesday, then yet another low pressure system will likely affect our region Wednesday. && .NEAR TERM /THROUGH TONIGHT/... Low pressure is located over Illinois this morning, moving northeast toward the Great Lakes region today. Fog has become a little more widespread across central Maryland and into northern Virginia. Based on observations and traffic cams, dense fog doesn't seem especially widespread. Have covered this with a short term forecast, although a short dense fog advisory is not out of the question. Expect more clouds than sun today under the influence of the warm advection regime ahead of the low pressure system. Some periods of sun are possible though, especially toward central Virginia. Most of the day will be dry across the entire area, but later this afternoon, some showers along a pre-frontal trough will approach the Appalachians. These should generally fall apart this evening as they outrun forcing and encounter downsloping east of the mountains. There could be some sprinkles though. Late tonight, a few showers could cross far southern Maryland as a secondary axis of moisture pushes northeastward. Temperatures will remain well above normal for mid December with highs in the mid 50s to mid 60s and lows in the 40s to lower 50s. && .SHORT TERM /SUNDAY THROUGH MONDAY NIGHT/... As the low moves into Quebec Sunday, the trailing cold front will cross the area. Forcing and moisture will be very limited along the front, so dry conditions are expected. Winds may be a bit gusty, but temperatures will remain mild (50s-mid 60s except along the Allegheny Front) since cold advection will be delayed. Some stratocumulus may prevent complete clearing, especially for locations north/west of Washington. Weak high pressure will nose in north of the area Sunday night. However, a compact southern stream trough will be driving an area of low pressure across the southeastern states. The low will reach North Carolina/southern Virginia by Monday morning. Precipitation will likely spread northward into the area very late Sunday night, with the bulk of the precipitation falling by early afternoon Monday. Potent lift from the shortwave and an upper jet streak will result in QPF amounts potentially exceeding an inch across southern parts of the CWA. There is still some model spread on the precipitation gradient, though the bulk of our CWA will likely get at least a little. Then the question turns to precipitation type. Temperatures/thermal profiles will be somewhat marginal, and forecast dew points in the lower to mid 30s don't allow a lot of room for wet bulb cooling either. Thus think this will be largely an elevation- driven event. Lower elevations could certainly see snow, though accumulations would likely be dependent on heavier rates/banding. It does appear like temperatures will be warm enough near/southeast of I-95 for just a chilly rain. Beyond that, I'm not terribly comfortable with the accumulation forecast in the mountains. The warm side of the envelope could result in marginal advisory (or less) accumulations, while a colder, mostly-snow solution could result in some fairly substantial totals along the ridgetops. Luckily it's still a few days away. Precipitation will quickly exit during the afternoon and temperatures will likely be able to warm into the 40s. Chilly northwest winds will bring most of the area below the freezing mark Monday night. && .LONG TERM /TUESDAY THROUGH FRIDAY/... Strong Canadian high pressure will build north of the Great Lakes Tuesday, shifting eastward toward Quebec and northern New England Tuesday night. The resultant weather locally will be dry conditions under a light northerly flow and below normal temperatures in the 30s to low 40s Tuesday. By Tuesday night, winds will turn more northeasterly as the high to the north migrates eastward. Clouds will be on the increase as well ahead of a broad area of low pressure extending from the northern Gulf Coast into the Tennessee Valley, as WAA aloft increases. Lows Tuesday night will be on the cold side thanks to the strong high to our north, with temperatures in the 20s for much of the area, hovering near 30 degrees in the cities and near the Bay. By Wednesday morning, an upper level trough will be digging south and east across the Mississippi River Valley, taking aim toward the TN/OH Valley's. The aforementioned area of low pressure is progged to consolidate over the eastern Carolinas Wednesday afternoon, strengthening as it tracks along or just east of the Delmarva Peninsula Wednesday night. At the same time, the upper trough will take on a negative tilt as it approaches the Ohio Valley late Wednesday and the right entrance of the jet positions itself nicely in terms of upward motion and divergence aloft centering over the CWA. Precipitation associated with the system will overspread the area Wednesday morning, a touch slower with the latest model runs, while much of the area is at or below freezing. This will allow for snow to the be the favored ptype initially. The model trends with the high over or near Quebec have been stronger, helping to supply the colder air on the eastern slopes of the mountains. Thereafter, guidance wants to allow for warmer air to work its way along and east of the I-95 corridor to produce a rain/snow mix Wednesday afternoon and evening. Further north and west, thermal profiles indicate an all snow event, with accumulating snow looking increasingly likely. Model ensembles support this as well, with fairly good agreement across the board, and probabilities of exceeding 1/3/6" all increasing run to run. At this time, an enhanced winter storm threat exists Wednesday and Wednesday night along and west of the Blue Ridge Mountains, potentially resulting in a disruption to travel, while a slight winter storm threat along and east of the I-95 corridor. With a touch slower progression of the system, models linger precipitation into a good portion of Wednesday night as the low pushes away from the coast Thursday morning. There is some discrepancy in this progression between the global guidance, so this will have to be ironed out in the coming days. Confidence is increasing with the potential for accumulating snow and potential impact to the area, but exact amounts and placement of the rain/snow line carries a lower confidence at this time. Follow our latest forecast on this system and potential impacts at weather.gov/lwx/winter. Upslope snow showers possible on the backside of the system across the Alleghenies Thursday as the rest of the area favors dry conditions and below normal temperatures through Thursday night as high pressure nudges back into the region. && .AVIATION /12Z SATURDAY THROUGH WEDNESDAY/... Low clouds and fog are slowly spreading across central MD at sunrise, primarily affecting DCA, BWI, and MTN. Localized LIFR conditions are occurring. Restrictions should part by mid-late morning, with VFR conditions and southerly winds 5-10 kt through tonight. A shower or sprinkle could cross the area tonight ahead of a cold front, but no impacts are expected. Based on review of additional model guidance, have added LLWS to the TAFs tonight as a low level jet passes overhead. However, it will be marginal and may depend on whether surface winds can become light. The front will cross on Sunday, and west winds could briefly gust to 20 kt. SubVFR conditions possible late Sunday night into Monday as low pressure tracks across the Carolinas, spreading precipitation over the terminals. MTN, BWI and DCA are more likely to just see rain, while IAD, MRB and CHO could encounter a mix of with snow. Still a lot of uncertainty with this system, although precipitation should exit during the afternoon. VFR conditions expected Tuesday and Tuesday night as high pressure dominates to the north. Low pressure will approach to our south and east Wednesday and Wednesday night, with sub VFR conditions becoming increasingly likely with wintry precipitation overspreading the terminals. && .MARINE... South winds will remain in place through tonight. Could see an increase in winds overnight, especially on the bay. Can't rule out SCA conditions, but am not confident enough to issue one at this time since air temperatures will be warmer than the water resulting in poor mixing. A cold front will move over the waters Sunday, shifting the wind to the west-northwest. At this time, sub-SCA conditions look to be the most likely scenario since warm temperatures will persist, but a brief period of 20 kt gusts will be possible. Low pressure passing to our south on Monday may yield SCA conditions over portions of the waters along with moderate rainfall. Northwest winds may continue to reach SCA levels through Monday night. High pressure north of the waters Tuesday and Tuesday night will allow for northerly winds to turn northeasterly, remaining light and sub SCA. Low pressure will track to the south and east of the waters Wednesday and Wednesday night, overspreading rain/snow across the area, and increasing the chances for SCA gusts over the waters as the gradient tightens. && .LWX WATCHES/WARNINGS/ADVISORIES... DC...None. MD...None. VA...None. WV...None. MARINE...None. && SYNOPSIS...ADS NEAR TERM...ADS SHORT TERM...ADS LONG TERM...BKF AVIATION...ADS/BKF MARINE...ADS/BKF
Hazardous Weather Outlook National Weather Service Baltimore MD/Washington DC 517 AM EST Sat Dec 12 2020 ANZ530>543-DCZ001-MDZ008-011-013-014-016>018-504-506-508- VAZ052>057-131030- Chesapeake Bay north of Pooles Island MD- Chesapeake Bay from Pooles Island to Sandy Point MD- Chesapeake Bay from Sandy Point to North Beach MD- Chesapeake Bay from North Beach to Drum Point MD- Chesapeake Bay from Drum Point MD to Smith Point VA- Tidal Potomac from Key Bridge to Indian Head MD- Tidal Potomac from Indian Head to Cobb Island MD- Tidal Potomac from Cobb Island MD to Smith Point VA- Patapsco River including Baltimore Harbor- Chester River to Queenstown MD-Eastern Bay- Choptank River to Cambridge MD and the Little Choptank River- Patuxent River to Broomes Island MD- Tangier Sound and the inland waters surrounding Bloodsworth Island-District of Columbia-Cecil-Southern Baltimore- Prince Georges-Anne Arundel-Charles-St. Marys-Calvert- Central and Southeast Montgomery-Central and Southeast Howard- Southeast Harford-Prince William/Manassas/Manassas Park-Fairfax- Arlington/Falls Church/Alexandria-Stafford-Spotsylvania- King George- 517 AM EST Sat Dec 12 2020 This Hazardous Weather Outlook is for the Maryland portion of the Chesapeake Bay, Tidal Potomac River, and I-95 corridor through central Maryland, northern Virginia, and District of Columbia. .DAY ONE...Today and Tonight No hazardous weather is expected at this time. .DAYS TWO THROUGH SEVEN...Sunday through Friday There is a slight winter storm threat for Wednesday and Wednesday night. If the threat materializes, it may cause travel disruptions. .SPOTTER INFORMATION STATEMENT... Spotter activation is not expected at this time. MDZ003>006-503-505-507-VAZ025>031-036>040-050-051-501-502-505>508- WVZ050>053-055-131030- Washington-Frederick MD-Carroll-Northern Baltimore- Northwest Montgomery-Northwest Howard-Northwest Harford-Augusta- Rockingham-Shenandoah-Frederick VA-Page-Warren-Clarke-Nelson- Albemarle-Greene-Madison-Rappahannock-Orange-Culpeper- Northern Fauquier-Southern Fauquier-Western Loudoun- Eastern Loudoun-Northern Virginia Blue Ridge- Central Virginia Blue Ridge-Hampshire-Morgan-Berkeley-Jefferson- Hardy- 517 AM EST Sat Dec 12 2020 This Hazardous Weather Outlook is for portions of eastern West Virginia, northern and central Virginia, and central and western Maryland. .DAY ONE...Today and Tonight No hazardous weather is expected at this time. .DAYS TWO THROUGH SEVEN...Sunday through Friday There is a slight winter storm threat for Monday. If the threat materializes, it may cause travel disruptions, including during the morning commute Monday. There is an enhanced winter storm threat for Wednesday and Wednesday night. The primary threat is disruption to travel. .SPOTTER INFORMATION STATEMENT... Spotter activation is not expected at this time. MDZ001-501-502-VAZ503-504-WVZ501>506-131030- Garrett-Extreme Western Allegany-Central and Eastern Allegany- Western Highland-Eastern Highland-Western Grant-Eastern Grant- Western Mineral-Eastern Mineral-Western Pendleton- Eastern Pendleton- 517 AM EST Sat Dec 12 2020 This Hazardous Weather Outlook is for the Allegheny Front counties in western Maryland, eastern West Virginia, and Highland County Virginia. .DAY ONE...Today and Tonight No hazardous weather is expected at this time. .DAYS TWO THROUGH SEVEN...Sunday through Friday There is a slight winter storm threat for Monday. If the threat materializes, it may cause travel disruptions, including during the morning commute Monday. There is an enhanced winter storm threat for Wednesday and Wednesday night. The primary threat is disruption to travel. .SPOTTER INFORMATION STATEMENT... Spotter activation is not expected at this time.
The national radar mosaic for the moment the first event began.
2 dense fog. Local standard time, as filed.
Dense fog developed early in the morning, with widespread visibility of 1/4 mile or below. Fog dissipated towards noon.
Dense fog developed early in the morning, with widespread visibility of 1/4 mile or below. Fog dissipated towards noon.
After NWS Directive 10-1605, the rules offices file Storm Data by.
Every storm in Virginia in 2020Every state in 2020This day in weather historyWarnings in Virginia, December 2020
NOAA National Centers for Environmental Information, Storm Events Database, compiled from the National Weather Service’s monthly Storm Data. Complete through 2026-06; NCEI adds each month about 75 days after it ends and revises past years a few times a year. Rebuilt 2026-09-24. Each event’s entry at NCEI: 935621, 935622.