Area Forecast Discussion
Issued by NWS Sterling, VA

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823 FXUS61 KLWX 261334 AFDLWX Area Forecast Discussion National Weather Service Baltimore MD/Washington DC 934 AM EDT Wed Jul 26 2017 .SYNOPSIS... High pressure will move offshore through tonight. An upper- level disturbance will pass through late Thursday into Thursday night. Potent low pressure will develop overhead Friday before slowly moving to the east Friday night through Saturday. The cold front associated with this low will pass through during this time. High pressure will return for early next week. && .NEAR TERM /THROUGH TONIGHT/...
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Interesting set-up this morning with scattered to numerous showers along portions of the western shore of the Chesapeake Bay including the Patapsco River and Lower Tidal Potomac River. Showers have been nearly stationary this morning resulting in a narrow corridor of wet conditions. KLWX is estimating over an inch of rainfall just southeast of Baltimore City. Showers are expected to persist through the morning hours before the sfc flow turns to the SE. Low stratus also expected across portions of northeast/central MD this morning. Further to the west...valley fog will continue to dissipate this morning and mostly sunny to partly cloudy conditions expected today. Previous discussion... Surface high pressure centered over the northeastern US this morning will shift offshore through the day today. A push of easterly flow carrying marine-influenced low level air looks to make it to about the I-95 corridor this morning before flow turns more southeasterly and mixing increases during the afternoon hours. Thus there is the potential for some low stratus and even a few rain showers from about I-95 eastward, especially near the Chesapeake Bay where warmer water temperatures have increased low level lapse rates. Elsewhere, some slightly higher-based strato-cu will be around along with some patchy fog. By the afternoon, any low stratus will be gone, having mixed out into a broken cumulus field. High temperatures from 80-85F.
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&& .SHORT TERM /THURSDAY THROUGH FRIDAY NIGHT/... Light southerly flow will continue tonight as high pressure elongates offshore before exiting and a system begins to approach from the west. There will be potential for some low clouds and patchy fog once again. Lows in the 60s to around 70F. Active period of weather is then expected for the end of the week, although timing and details are still uncertain. Main shortwave trough will be entering the northern Plains Thursday morning with a leading shortwave and surface low pressure/front ejecting from the Mississippi Valley at the same time. This leading wave/front will move into the Ohio Valley during the day Thursday with warmth and humidity returning out ahead of it. Highs on Thursday will rise back up into the upper 80s to around 90F with dew points rebounding to near 70F. While mid-level lapse rates will be quite poor, 500-1000 J/KG of MLCAPE is still likely to develop, and with 0-6KM shear of 30-40 knots moving over top, scattered showers/thunderstorms still look to develop, especially west, with a severe threat. SPC has maintained the slight risk of severe thunderstorms. Precipitable water values will also surge to near or above 2 inches during this timeframe, so heavy rain and flood risk will also need to be monitored. For Thursday night, this leading wave/frontal boundary will push near the area and stall before the primary stronger shortwave approaches on Friday. This will induce cyclogenesis along the boundary on Friday with the surface low tracking near the region later Friday and Friday night. Depending on low track/placement, an additional, perhaps greater, threat of heavy rain, flash flooding, and severe thunderstorms exists for Friday. It should be noted that significant model differences continue to exist and confidence in any particular outcome at this time is relatively low. && .LONG TERM /SATURDAY THROUGH TUESDAY/... Low pressure will slowly move off to the east Saturday and the cold front associated with the low will drop well to our south. The timing of this system remains uncertain...but given slower trend in guidance over the last 24-hours and the fact that the upper-level low will be closed off...the latest forecast leans closer to the slower solutions. Therefore...unsettled conditions are likely Saturday morning before gradually improving during the afternoon and evening hours from west to east. Northerly flow behind the cold front will usher in noticeably cool conditions for this time of year Saturday. Max temps may hold in the 70s for most locations. High pressure will return for Sunday...bringing more cool conditions for this time of year along with low humidity thanks to a northwest flow and upper-level trough remaining overhead. However...more sunshine should allow for max temps to be warmer compared to Saturday. The upper-level trough will weaken some for Monday and Tuesday...but weak high pressure will likely remain nearby. Seasonably warm conditions are expected during this time...and most of the time should be dry. Will have to watch because high pressure will be strengthening over the Atlantic during this time. This has the potential to usher in subtropical moisture which would enhance chances for precipitation. However...as of now it does appear more likely that weak high pressure nearby should keep this to our south and east. && .AVIATION /13Z WEDNESDAY THROUGH SUNDAY/... Potential for morning MVFR stratus exists, mainly BWI/MTN/MRB along with some patchy MVFR/IFR fog/br at MRB. This will improve by the afternoon with a broken strato-cumulus deck area-wide with cloud bases 3500-5000 feet. Light east/northeasterly flow this morning will then turn southeasterly by the afternoon. For tonight, there will be a returning chance for patchy fog and low stratus, again mainly MRB/BWI/MTN. By Thursday, south/southwest flow returns along with increasing chances for showers/thunderstorms late in the day. Showers and possible thunderstorms become more widespread on Friday with periods of sub-VFR conditions possible. Subvfr conditions are possible Saturday morning along with showers. However...conditions should gradually improve during the afternoon and evening hours. High pressure will build overhead for Sunday. && .MARINE...
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Update...Coverage of showers have expanded across the western shore of the Chesapeake Bay and Lower Tidal Potomac River this morning. Brief reductions in visibilities are possible. Main marine-related weather concern this morning will be the potential for an isolated waterspout. A weak low level boundary associated with a westward push of marine-influenced air will be near the waters this morning. Low level lapse rates have steepened due to the warmer water temperatures and despite weak shear profiles, isolated showers this morning will have the potential to produce a waterspout. Low pressure will gradually move away from the waters Saturday and high pressure will build overhead for Sunday and Monday. A Small Craft Advisory will likely be needed for the waters Saturday through Saturday night...and may be needed Sunday. A Gale Warning may even be needed for portions of the waters Saturday into Saturday evening.
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&& .TIDES/COASTAL FLOODING... An onshore flow will develop today and persist through Thursday. Elevated water levels are expected during this time...but since the flow is light minor flooding is not expected at this time. Potent low pressure will approach the waters Thursday night before slowly passing through later Friday into Saturday. Increasing southerly surface winds ahead of the low may enhance the threat for minor flooding Thursday night through Friday...and possibly into Friday night. Confidence is low due to low certainty on the exact track of the low. && .LWX WATCHES/WARNINGS/ADVISORIES... DC...None. MD...None. VA...None. WV...None. MARINE...None. && $$ SYNOPSIS...BJL NEAR TERM...MM/HSK SHORT TERM...MM LONG TERM...BJL AVIATION...BJL/MM MARINE...BJL/MM/HSK TIDES/COASTAL FLOODING...LWX

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