Area Forecast Discussion
Issued by NWS Sterling, VA

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000 FXUS61 KLWX 120249 AFDLWX Area Forecast Discussion National Weather Service Baltimore MD/Washington DC 949 PM EST Thu Jan 11 2018 .SYNOPSIS... High pressure will continue drifting east of the area tonight as low pressure develops in the Tennessee Valley. This low will pass near the area by Friday night, followed by a cold front sweeping eastward early Saturday morning. High pressure of Canadian origin will build across the region through early next week. && .NEAR TERM /THROUGH FRIDAY/...
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High pressure continues to drift east of the area this evening. The leading edge of light rain/low clouds is now working across the CWA. A 40-50 kt low level jet will result in strong theta-e advection, which should result in steady or rising temperatures through the night. The isentropic lift will also bring occasional rain. Precipitable water values potentially exceeding 1.5 inches (record territory for this time of year) may result in locally moderate to heavy rain elements. If an isolated rumble of thunder is possible, it`d likely be over southern MD...but still think the chance was too low to include in the forecast. Since dewpoints will be rising in concert with the temperatures, foggy/soupy conditions are likely to slowly develop, especially as rain finally falls across the region and moistens the low levels fully. Mixing will be poor, limiting surface wind gusts, but it has become breezy in some locations. This first piece of energy will be lifting to the northeast Friday morning. There may be a lingering band of rain near the Bay before it moves off to the east. There will still be copious moisture in deep layer southerly flow, so some showers can`t be ruled out at any point of the day, but they will be lighter and some dry breaks are likely. This should allow a little mixing to occur, which will allow temperatures to rise into the mid 60s in many locations. Have stayed on the conservative end of guidance though since low level clouds look to stay locked in. That said, if there is any sun at all, there will be a shot at 70.
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&& .SHORT TERM /FRIDAY NIGHT THROUGH SATURDAY NIGHT/...
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Guidance has continued to trend eastward with the track of low pressure crossing the area...now directly crossing the CWA. Shower coverage will increase during this time. A convective line may develop ahead of the associated cold front. There will be plenty of wind energy and perhaps a little instability. Forecast has not changed substantially here, with a slight chance of thunder. If the line is well-developed, there could be strong winds associated with it. Despite a resurge in precipitable water ahead of the front, heavy showers should be transient. Additional hydrology thoughts are below. An anafrontal structure will be present with this system, resulting in crashing surface temperatures (20+ degrees in a matter of hours) while warmer air remains aloft. The NAM and GFS are rather quick with exiting most of the precipitation before colder surface air arrives, while the ECMWF seems to have strong frontogenetical forcing aloft, leading to precipitation hanging on longer, especially in the mountains. Have continued a conservative forecast approach for now, with an inch or two of snow in the mountains and a chance of mix Saturday morning to the east before ending. It will likely be windy behind the front too...will have to keep an eye on how strong. Probably not wind advisory, but might get close. Some clouds may linger into Saturday afternoon, but even in the mountains, any precipitation should be quickly ending. Temperatures will be holding steady or falling further, which will be below freezing in the west and in the 30s to the east. By Saturday night, lows will be falling into the teens to lower 20s with wind chills in the single digits.
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&& .LONG TERM /SUNDAY THROUGH THURSDAY/...
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High pressure will keep cool and dry conditions Sunday into Monday as it tracks northeast away from our region. High temperatures Sunday and Monday will be in the 20s and 30s... teens at higher elevations. Lows in the teens and single digits... some 20s Monday night. Upper level troughing will settle over the eastern CONUS during this time. Guidance suggests that a clipper system will be approaching our area Monday night into Tuesday. How much moisture and how fast it moves over our region is still the question. The GFS seems drier and faster than the Euro... which also develops a coastal low that eventually moves NNE away from us. Therefore the potential for snow over our area is increasing for early next week. Upslope snow looks more likely Monday night into Thursday. For the rest of our CWA there is still uncertainty.
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&& .AVIATION /03Z FRIDAY THROUGH TUESDAY/...
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Low clouds and light rain continue spreading north this evening. IFR still looks likely later tonight for both cig and vsby as moisture increases, with a period of moderate rain possible. In addition, a low-level jet of 40-50 kt will lead to widespread wind shear through tonight. There should be some breaks in the showers Friday, but can`t totally rule out rain at any time. Conditions could return to MVFR later in the day, but there is a potential that it won`t, especially near Baltimore. Then, a cold front crosses late Friday night. Another round of poor flight conditions in heavy rain (maybe even a rumble of thunder?) and gusty winds will result. Flow will be northwest before sunrise Saturday morning, and will remain so through the day. Ceilings will lift, but gusty winds (30 kt or so) will persist through the day. VFR conditions expected between Sunday and Monday with high pressure over our area. Sub-VFR conditions possible Monday night into Wednesday with possible snow over our area.
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&& .MARINE...
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Winds will increase over the waters as we head through the night. Even though cold water will result in poor mixing, the magnitude of wind available just off the deck suggests that Small Craft Advisory criteria will still be reached. Expanded SCA a bit given the strong LLJ just off the deck. Think the current extent likely covers it, but might end up having to add the Potomac at some point. As the low level jet departs Friday, there should be a bit of a lull Friday afternoon (at least below threshold), before winds increase again Friday night ahead of a cold front. SCA has been confined to the wider parts of the Bay Friday evening, expanding to all waters later at night as the front crosses. Gales are certainly possible behind the front into Saturday morning, but have held off for now due to marginal nature and timing uncertainty. SCA likely continue into Saturday night. Winds will be below small craft criteria Sunday into Tuesday. Therefore not expecting advisories during these days.
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&& .HYDROLOGY... Warming conditions are expected through the end of the week, along with periods of rain tonight through Friday night. Break up of ice may cause localized issues, but confidence is currently too low and threat too isolated to include in the HWO at the moment, but we will continue to monitor. Around one to one and a half inches of rainfall is anticipated during this period, subject to change based on finer scale details yet to be ironed out. While precipitable water is well above climatology, suggesting that flooding is possible, relatively quick movement of individual elements and dry antecedent conditions should preclude a widespread hydrologic issue. And even though soils are warming, absorption will still be limited, thus substantial ponding is possible, but not enough issues are expected to need a Flood Watch at this time. && .CLIMATE... Meteorological winter (which began December 1st) has been very dry so far. Below is a list of the top 5 driest meteorological winters (December 1st through February 28th/29th) on record. Washington DC area (DCA) 1. 2.60 inches (1871-72) 2. 3.32 inches (2001-02) 3. 3.85 inches (1980-81) 4. 4.15 inches (1976-77) 5. 4.76 inches (1873-74) Notes: The winter of 1870-71 had 4.16 inches, but 31 days of data are missing (no data for December 1870). So far, Winter 2017-18 has had 0.66 inches through January 8th. Precipitation records date back to 1871. Baltimore MD area (BWI) 1. 4.03 inches (1976-77) 2. 4.12 inches (1980-81) 3. 4.28 inches (2001-02) 4. 4.30 inches (1871-72) 5. 4.51 inches (1979-80) Notes: The winter of 1870-71 had 2.93 inches, but 31 days of data are missing (no data for December 1870). So far, Winter 2017-18 has had 1.07 inches through January 8th. Precipitation records date back to 1871. Dulles VA area (IAD) 1. 3.24 inches (2001-02) 2. 3.37 inches (1976-77) 3. 4.83 inches (1979-80) 4. 5.18 inches (1980-81) 5. 5.55 inches (2010-11) Notes: The winter of 1962-63 had 0.00 inches, but 38 days of data are missing. The winter of 1961-62 had 5.34 inches, but 34 days of data are missing. So far, Winter 2017-18 has had 0.80 inches through January 8th. Precipitation records date back to 1960. && .LWX WATCHES/WARNINGS/ADVISORIES...
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DC...None. MD...None. VA...None. WV...None. MARINE...Small Craft Advisory until noon EST Friday for ANZ530>533- 537>542. Small Craft Advisory from 6 PM Friday to 6 AM EST Saturday for ANZ531>534-537-539>541-543. Small Craft Advisory from midnight Friday night to 6 AM EST Saturday for ANZ530-535-536-538-542. Small Craft Advisory until noon EST Friday for ANZ534-543.
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&& $$ SYNOPSIS...ADS NEAR TERM...ADS/RCM SHORT TERM...ADS/RCM LONG TERM...IMR AVIATION...ADS/IMR/RCM MARINE...ADS/IMR/RCM HYDROLOGY...ADS CLIMATE...DHOF

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