Area Forecast Discussion
Issued by NWS Sterling, VA

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000 FXUS61 KLWX 191905 AFDLWX Area Forecast Discussion National Weather Service Baltimore MD/Washington DC 305 PM EDT Fri May 19 2017 .SYNOPSIS...
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A cold front will push south through the area tonight. The boundary will stall out to the south later Saturday. The boundary will return north as a warm front into our area late Sunday before a stronger cold front passes through from the west Monday. High pressure will briefly return for Tuesday before another cold front impacts the area Wednesday.
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&& .NEAR TERM /THROUGH TONIGHT/...
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Bermuda high pressure remains in place this afternoon with hot and humid conditions, although a surface trough bisects the area, while a cold front is positioned just south of the Great Lakes. There is plenty of instability this afternoon (up to 2000 J/kg per mesoanalysis), but very little shear (20-25 kt). Scattered thunderstorms are initiating across the area, and this trend will continue though the afternoon. There are several foci for storm development, including the surface trough, a second subtle wind shift near the PA border, and an MCV in southern VA. At this time the strongest storms are generally expected along and south of the surface trough where winds are backed to the southwest (since deep westerly winds often suppressconvection), although a strong pulse storm could occur just about anywhere due to the instability. Storms which merge into clusters could also pose a marginally severe threat. Hail and locally damaging winds will be the main threats. Most hi-res guidance shows the primary clusters of storms exiting to the east during the early evening. The cold front then drops southward overnight. There may also be a shortwave trough (which could be impacted by Ohio Valley convection this afternoon) crossing during this time. Thus a renewed/continuing shower threat may exist for much of the night. While thunder chances will be less, there could be a few storms as well with some lingering elevated instability. Frontal passage will bring lows in the 50s to the northern quarter of the area, while the rest stay in the 60s.
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&& .SHORT TERM /SATURDAY THROUGH SUNDAY NIGHT/...
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The front will stall south of the area Saturday, and possibly remain there well into Sunday. Onshore flow will develop, leading to cloudier and cooler conditions. Model guidance differs on the thickness of cloud cover and thus temperatures. Have high temperatures on either side of 70 both days, but it could be cooler if thick, low clouds develop. Precipitation chances will be limited for Saturday through Sunday morning. There is a better chance of showers across western parts of the area due to upslope and overrunning flow. Depending on the position of the warm sector, thunderstorms could also develop Saturday in Highland/Pendleton Counties. Should any of this activity try to move eastward, it will quickly weaken in the stable airmass. A low level jet will increase ahead of an approaching cold front Sunday afternoon into Sunday night. It`s uncertain how much the warm front lifts back north, but nevertheless, forcing and moisture will be there for increasing chances of rain, especially during the second half of Sunday night. There could be a rumble or two of thunder with elevated instability.
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&& .LONG TERM /MONDAY THROUGH FRIDAY/...
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A cold front will move across the region Monday bringing increased chances of thunderstorms that could produce heavy downpours. Dry air returns with a separate area of high pressure building in from the west Monday night through Tuesday. A low pressure system over the lower Great Lakes will intensify as it meanders eastward Tuesday night through Wednesday night. There is a chance for showers and perhaps a thunderstorm or two as pieces of energy rotate around the main storm system. A breezy and dry day expected Thursday, mainly to the east of the Potomac Highlands, with high pressure building in from the west. A few rain showers are possible in the Potomac Highlands. A piece of upper level energy could bring a chance for a few showers Friday.
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&& .AVIATION /19Z FRIDAY THROUGH WEDNESDAY/...
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VFR conditions are expected through tonight for most of the time outside of thunderstorms. Coverage will peak during the afternoon to early evening, though a direct impact to a terminal is around 50-60 percent. Placed TEMPO -TSRA group for DCA/BWI where models focus developing cluster along a surface trough, although amendments are likely for all terminals. Gusty winds, heavy rain, and hail will be possible. Cold front will push south tonight. Some showers may develop along the front but impacts are uncertain. It is also possible some fog tries to develop ahead of the front, but did not have confidence to put in the TAFs due to mostly cloudy skies. Onshore flow will develop Saturday through Sunday. The marine air will increase the chances for low clouds...especially later Saturday night and Sunday. Models differ on if MVFR clouds form Saturday, so have placed this possibility in a SCT group (except CHO where there is better agreement). More low clouds are likely Sunday night along with increasing chances for rain ahead of a cold front. IFR conditions possible at all terminals Monday. Winds southwest becoming northwest 10 knots. VFR conditions Monday night and Tuesday. Winds northwest 5 knots Monday night becoming southeast 5 knots Tuesday. MVFR conditions Tuesday night with a chance of showers. Winds south becoming west 5 to 10 knots.
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&& .MARINE...
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Bermuda high will continue to influence the weather pattern through today. West to southwest winds will continue through this evening 10-15 kt. Scattered thunderstorms will reach the waters late this afternoon and evening and could contain strong winds and hail. A cold front will pass through the waters later tonight. The pressure surge with the cold front will cause north to northeast winds behind the boundary to gust around 20 knots. A Small Craft Advisory is in effect for the waters later tonight into Saturday morning. Have extended the SCA into the afternoon for much of the Chesapeake Bay and lower Tidal Potomac, as the pressure gradient looks strong enough to keep winds elevated. Wind fields should decrease Saturday night. The onshore flow will continue through Sunday before turning southerly Sunday night ahead of an approaching cold front. A Small Craft Advisory may be needed for portions of the waters Sunday night. The front will cross the area Monday, with the gusty wind threat decreasing. Lighter winds Monday night through Tuesday night with high pressure.
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&& .TIDES/COASTAL FLOODING...
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Onshore flow will develop this weekend, turning southerly by Sunday night. Water levels will increase, with anomalies supporting a minor coastal flood threat at Annapolis and Straits Point by Sunday night.
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&& .CLIMATE...
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Unseasonably hot conditions continue today. Record heat is not anticipated for BWI or DCA today...but it is possible for KIAD. Here is a list of record high temperatures and record high minimum temperatures for today...May 19th. Friday (05/19) Record Warm Lows: DCA: 72 in 2015 BWI: 75 in 1877 IAD: 66 in 2015 Friday (05/19) Record Highs: DCA: 96 in 1997 BWI: 98 in 1962 IAD: 92 in 1997
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&& .LWX WATCHES/WARNINGS/ADVISORIES...
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DC...None. MD...None. VA...None. WV...None. MARINE...Small Craft Advisory from 11 PM this evening to 6 PM EDT Saturday for ANZ531-539. Small Craft Advisory from 11 PM this evening to 11 AM EDT Saturday for ANZ530-538. Small Craft Advisory from 1 AM to 6 PM EDT Saturday for ANZ532>534-536-537-540>543. Small Craft Advisory from 1 AM to 11 AM EDT Saturday for ANZ535.
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&& $$ SYNOPSIS...ADS NEAR TERM...ADS SHORT TERM...ADS LONG TERM...KLW AVIATION...ADS/KLW MARINE...ADS/KLW TIDES/COASTAL FLOODING...ADS CLIMATE...ADS

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