Area Forecast Discussion
Issued by NWS Sterling, VA

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000 FXUS61 KLWX 071905 AFDLWX Area Forecast Discussion National Weather Service Baltimore MD/Washington DC 305 PM EDT Fri May 7 2021 .SYNOPSIS... Low pressure and its associated cold front will cross the area this afternoon. A secondary cold front will move through the area Saturday. High pressure will build toward the region Saturday night. Another cold front will cross the area early on Monday. High pressure will return for the middle portion of next week. && .NEAR TERM /THROUGH TONIGHT/...
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While a broad shield of thickening mid to high clouds have filled out the region, some pockets of 250-500 J/kg surface- based CAPEs have emerged. Within this tongue of instability sprawled across the I-95 corridor, a series of convective showers have fired. Recent lightning data has a handful of strikes across Prince William County and Prince Georges Counties within the more robust cells. Based on radar trends and high- resolution model data, showers may occasionally pulse up from time to time into the evening hours. With the 12Z KIAD sounding showing modest mid-level lapse rates, cannot rule some pea-sized hail or so within the stronger updrafts. Precipitable waters remain close to climatology which would offset any notable flooding risk. The eastward advancing cold front will keep showers in play through the evening hours with total rainfall amounts likely in the 0.25-0.75 inch range. This batch of precipitation should exit out of the area by later this evening. Within the broad cyclonic flow filling much of eastern North America, a secondary upper low begins its east-southeastward progression toward the Appalachians during the overnight. Modest ascent ahead of this trough will bring precipitation back to the Alleghenies. Given the seasonably cool air mass in place, some of the higher elevations may see a rain/snow mix. With the growing season already underway, this could impact some outdoor vegetation. Locations farther downstream are to stay mostly dry overnight with temperatures running in the upper 30s to low 40s.
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&& .SHORT TERM /SATURDAY THROUGH SUNDAY NIGHT/...
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The mentioned upper low is forecast to track across the region during the morning to early afternoon hours. For the most part, this will not be one to one with the peak in diurnal heating. In spite of this, numerous pop-up showers are likely to fire as the trough passes overhead. Some small hail or graupel may fall out of the stronger convective showers given the degree of cold air aloft. Expected 1000-500 mb thicknesses should drop below 534 dm which is very impressive for the time of year. As this system begins to swing offshore, gusty northwesterly winds up to 25 to 30 mph are possible in the wake. With forecast highs only in the upper 50s to low 60s, departures from average are in the 10 to 15 degree range. This will be followed by another chilly night well down into the 40s with near freezing temperatures on the peaks of the Alleghenies. On Sunday, a surface low is slated to track from Missouri toward western Pennsylvania. While doing so, it should continue to weaken as the mid-level vorticity centers begin to shear out. The main focus will be on a band of isentropically driven rain set to fall well north of a warm front across the southern Mid- Atlantic. Model guidance continues to trend northward with the heavier axis of rainfall, mainly north of the Mason-Dixon Line. This would also carry any inherent flooding threat north, but will continue to monitor for any future southward trends. Overall, Sunday (Mother`s Day) should be unsettled across the area with a notable temperature gradient from north to south. Closer to the Pennsylvania border, highs will be in the 50s, while readings in central Virginia could reach the mid 70s. Heading into the nighttime, milder conditions are in store with the increase in warm advection and cloud cover.
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&& .LONG TERM /MONDAY THROUGH THURSDAY/...
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A weak cold front will move through the region Monday afternoon. Instability and wind shear look marginal overall, with the local maxima over the far southeast portions of our CWA. Therefore, cannot rule out a thunderstorm with the frontal passage, though showers are favored. A notable gradient in high temps will exist Monday, with highs near 50-60 over the far northwest portions of the CWA, and highs in the mid to upper 70s across the far southeast zones. An area of Canadian high pressure will make for a period of unseasonably cool weather Monday night through Tuesday night before shifting offshore on Wednesday. With 850 temps near or below freezing, freeze concerns become likely Monday and Tuesday night across the higher terrain of the Allegheny Front. East of there, frost conditions are possible as far east as the Route 15 corridor if the winds are low enough. A brief upper-level ridge will build overhead Wednesday raising heights and increasing temps a few degrees, though still below normal highs for mid-May. Wednesday likely stays dry. A mid to upper level shortwave will move from the Central Plains into the Ohio Valley Wednesday night before spreading east into the local area Thursday into Friday. Rain returns as early as Wednesday night and could linger into Friday as an area of low pressure from the southern stream approaches. Lowered tempers Thursday to the 25th percentile of NBM given overall pattern and most guidance being on the cool side.
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&& .AVIATION /18Z FRIDAY THROUGH WEDNESDAY/...
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Areas of showers will continue to move from west to east across the terminals through the mid-evening hours. Some of the stronger cores may produce isolated lightning strikes as noted in portions of Prince William and Prince Georges County. However, the environment is not conducive for any widespread thunderstorm threat. MVFR ceilings/visibility are possible through about 02Z before these showers begin to exit off to the east. VFR conditions should generally be maintained for the weekend with gusty northwesterly winds for Saturday behind another cold front. Gusts could approach 25 knots or so by the afternoon hours. This upper low will also produce scattered showers from the late morning into the afternoon. Winds turn to southerly on Sunday with a chance for showers through the day. Brief conditions under VFR are possible given the persistent shower threat. VFR conditions likely Monday ahead of the cold front. As the front moves through, could see MVFR conditions due to showers or thunderstorms. The winds increase briefly out of the northwest Monday evening behind the front, but conditions remain VFR on Tuesday.
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&& .MARINE...
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Initially, small craft conditions will be restricted to southern portions of the Tidal Potomac and Chesapeake Bay beginning later this evening. By Saturday morning, all the waters can anticipate such conditions given the gusty northwesterly winds. This will easily continue into Saturday night as well as Sunday. With the latter, winds will have turned to southerly in response to the warm advection. On the precipitation side, numerous showers are to impact the waterways through this evening. An isolated thunderstorm cannot be ruled out as well. Additional showers loom into the weekend. SCA conditions are likely Monday with the cold front passage and could linger into Tuesday. Mariners should beware of the potential for some showers and storms Monday afternoon that could produce gusty winds.
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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 AM to 6 PM EDT Saturday for ANZ530>533-535-536-538>542. Small Craft Advisory from 10 PM this evening to 6 PM EDT Saturday for ANZ534-537-543.
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&& $$ SYNOPSIS...LFR/BRO NEAR TERM...BRO SHORT TERM...BRO LONG TERM...CPB AVIATION...BRO/CPB MARINE...BRO/CPB

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