Area Forecast Discussion
Issued by NWS Sterling, VA

Home | Current Version | Previous Version | Text Only | Print | Product List | Glossary On
Versions: 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50
000
FXUS61 KLWX 210929 AAA
AFDLWX

AREA FORECAST DISCUSSION...UPDATED
NATIONAL WEATHER SERVICE BALTIMORE MD/WASHINGTON DC
529 AM EDT THU AUG 21 2014

.SYNOPSIS...
A STATIONARY FRONT WILL WAVER ACROSS THE MID ATLANTIC AS RIPPLES
OF LOW PRESSURE MOVE ALONG THE BOUNDARY. THE LAST IMPULSE WILL
TRAVERSE THE FRONT FRIDAY NIGHT...WHICH WILL PUSH IT SOUTH AS A
COLD FRONT. HIGH PRESSURE FROM NEW ENGLAND WILL DOWN DOWN THE EAST
COAST THIS WEEKEND.

&&

.NEAR TERM /THROUGH TONIGHT/...
UPDATE...AN EARLIER SPEC WX STATEMENT FOR LOCALLY DENSE FOG HAS
BEEN REPLACED W/ A DENSE FOG ADZY FOR MUCH OF THESE SAME AREAS...
ESPEC A TIER OF COUNTIES EITHER SIDE OF THE NRN VA BLUE RIDGE/MD
CATOCTINS. THIS INCLUDES PORTIONS OF THE NRN SHEN VLY AS WELL.
RAIN FROM LAST EVE...CALM WINDS AND A MOIST BOUNDARY LAYER
CONTRIBUTING TO AN INCREASINGLY DENSE - THOUGH SHALLOW - LOW
STRATUS LAYER OVER THE MTN VLYS AND PIEDMONT REGIONS.

PREV DISC...
THE UPPER-LEVEL LOW PRESSURE PINWHEEL A FEW HUNDRED MILES TO OUR
NORTH WILL SLINGSHOT ANOTHER WAVE OF MOISTURE ACROSS THE REGION
LATER TODAY...BEFORE DISSIPATING AND BEING BECOMING PART OF A
LARGER UPPER TROUGH. OUR MOIST AIRMASS WILL UNDERGO VERY LITTLE
CHANGE OVER THE NEXT COUPLE OF DAYS. THIS MODIFIED SUB-TROPICAL
REGIME HAS A CORRIDOR OF DRY AIR ATOP A DENSE AND VERY MOIST
BOUNDARY/SFC LAYER. NW FLOW ALOFT WILL CARRY WAVES OF EMBEDDED
VORT MAXES...WHICH ARE ESSENTIALLY AREAS OF LIFT...BUT IN AN
INTERMITTENT AND RANDOM NATURE. LATER TODAY/THIS EVE...A MORE
COMPACT AREA OF UPPER ENERGY WILL BE SHOVED QUICKLY DOWN ACROSS
THE UPPER MIDWEST AND OVER THE MID-ATLC REGION.

LIKE WED HOWEVER...THE UPPER LOW WILL FORCE THIS WAVE DOWN TOWARD
THE AREA BUT KIND OF STABLE COASTAL BLOCK WILL DISSIPATE THE ENERGY
AS IT APPROACHES THE ATLC COASTLINE. BUT BEFORE THEN...PLENTY OF
MOISTURE AND MARGINAL INSTABILITY WILL BE AVAILABLE FOR THE INCOMING
SHORTWAVE ENERGY TO DEVELOP CONVECTION. ONE OF THE KEY FACTORS WILL
BE OUR HIGH TEMPS...AGAIN EXPECTED TO REPEAT IN THE M-U80S. THESE
VALUES COMBINED W/ SFC DEWPOINTS IN THE U60S/L70S WILL BE MORE THAN
ENOUGH TO AT LEAST BREAK THE WEAK CAP AND FORCE FOR THE
CONDITIONALLY UNSTABLE ATMOS TO DEVELOP SCATTERED SHOWERS/TSTMS. A
HANDFUL OF ISOLATED SHOWERS COULD DOT THE MAP OVER THE COMING
HRS...BUT NOT UNTIL MID-LATE AFTN IS MORE ORGANIZED/WIDESPREAD
DEVELOPMENT EXPECTED. TIMING/LOCATIONS/INTENSITIES WILL DEPEND ON
EARLY OUTFLOWS AND STORM MOMENTUM FACTORS BUT THE LOW LEVEL SHEAR
WILL CERTAINLY BE ENOUGH /20-30KT BOUNDARY LAYER/ TO SUSTAIN A
LINEAR FEATURE.

AS W/ THE CONVECTION ON WED AFTN/EVE...THE ACTIVITY LEVEL WILL
QUICKLY DROP OFF AFT SUNSET AND W/IN A COUPLE HRS AFTERWARD.
OVERNIGHT LOWS WILL DROP ONLY INTO THE U60S/L70S AGAIN SINCE
DEWPOINTS WON`T BUDGE - SETTING UP FOR ANOTHER WAVE OF SHOWERS/TSTMS
ON FRI AFTN.

&&

.SHORT TERM /FRIDAY THROUGH FRIDAY NIGHT/...
MOST SHORT/MED RANGE MODELS...INCLUDING THE TAIL END OF SOME LOCAL
WRF MEMBERS ARE IN GOOD AGREEMENT ON A FRI AFTN AND INTO LATE EVE
HRS HEIGHTENED LEVEL OF SHOWER/TSTM ACTIVITY ACROSS PORTIONS OF THE
AREA. POPS ARE HIGHER THAN THE GARDEN-VARIETY ISOLATED AFTN REGIME.
THIS COULD MEAN THAT A MORE STRATIFORM/DISSIPATED REGION OF EARLIER
CONVECTION WILL DROP A REGIONAL-SCALE BATCH OF LIGHT/MODERATE RAIN
OVER THE AREA...ESPEC ACROSS THE WRN HALF OF THE CWA AS MODELS
SUGGEST.

INCREASING CLOUD COVER FROM LATE TONIGHT INTO EARLY FRI COULD ALSO
INHIBIT CONVECTIVE POTENTIAL AND LIMIT IT TO AREAS FURTHER S AND E
WHERE TEMPS ARE EXPECTED TO PEAK IN THE M80S. MODEL ADJUSTED VALUES
ARE GIVING L80S FOR HIGHS...ACCOUNTING FOR THE CLOUD COVER
INCREASE...A LIGHT NLY WIND THAT IS EXPECTED TO TURN MORE ELY OVER
TIME AND CERTAINLY INTO THE LATE NIGHT HRS - WHICH IS A MORE STABLE
FLOW.

&&

.LONG TERM /SATURDAY THROUGH WEDNESDAY/...
H5 PTTN WL BE EVEN MORE AMPLIFIED IN THE XTNDD PD...W/ RDGG IN THE
CENTER OF THE CONUS AND TROFFING ALONG THE ERN SEABOARD. THAT WL
PERMIT SFC HIPRES OVER THE CNDN MARITIMES AND DOWNEAST MAINE TO BLD
DOWN THE COAST. THEREFORE...AM BCMG INCRSGLY CONFIDENT THAT THE WET
PTTN OF THIS WK WL BREAK.

SAT WL BE THE TRANSITION DAY. WL STILL HV ELY SFC LOW AND MODEST
INSTBY. MEANWHILE...THE JET STREAM WL STILL BE OVHD...PROVING A ZONE
OF SHEAR. WITH A MARITIME AMS IN THE LLVL AND LIFT W/IN THE
COLUMN...NEED TO PAINT A GLOOMY PICTURE. WL BE HOLDING POPS AT
CHC...AND EMPHASIZING POINTS E OF THE BLURDG /UPSLP FLOW/. AM NOT
GOING AS LOW ON MAXT AS PSBL. SINCE AM NOT GOING FOR A WASHOUT...WL
BE HOLDING OUT SOME HOPE THAT THERE WL BE SOME OPPORTUNITY FOR TEMPS
TO RISE FM DEWPTS. THOSE DEWPTS SHUD BE UPR 60S TO NEAR 70F...SO
ITLL BE A MUGGY DAY EVEN W/O SUNSHINE.

WL BE PUSHING PCPN BACK INTO THE MTNS SAT NGT-SUN AS RDGG BLDS.
STILL HV PROLONGED ELY FLOW...SO SUN WL BE THE DAY WHEN AIR TEMPS WL
BE STRUGGLING TO RISE ABV WATER TEMPS. THERE MAY BE ENUF AIR MVMT SO
THAT FOG WONT BE AN ISSUE SAT NGT. IF IT TURNS OUT THAT FLOW DCPLS
INSTEAD...IT WL BE A WHOLE DIFFERENT STORY.

SFC HIPRES WL RUN THE SHOW FOR THE REST OF THE XTNDD. AM MUCH MORE
GENEROUS W/ SUNSHINE. MAXT UPR 70S-LWR 80S/MIN-T 50S TO LWR 60S
STILL BELOW CLIMO...BUT REACHING TIME OF YR WHEN NRMLS STARTING TO
DROP.

&&

.AVIATION /10Z THURSDAY THROUGH MONDAY/...
GROUND/VLY FOG HAS SETTLED OVER PARTS OF THE REGION...ESPEC ALONG
THE NRN VA PIEDMONT IN TYPICAL FOG PRONE VLYS AND AREAS THAT
RECEIVED RAIN LAST EVE. THE METRO AREAS WILL ONLY SEE A BRIEF PERIOD
OF HIGH-END VSBY GROUND FOG AND POTENTIALLY A BRIEF MVFR/IFR STRATUS
DECK FROM THE MOIST OVERHEAD ENVRMNT TOWARD SUNRISE. SKIES AFTER
SUNRISE WILL WAVER IN/OUT OF PARTLY SUNNY/MOSTLY CLOUDY SINCE WE
STILL HAVE A MOIST ATMOS. AN UPPER-LEVEL DISTURBANCE WILL SWING
ACROSS THE REGION LATER TODAY...POTENTIALLY CARRYING ANOTHER WAVE OF
SHOWERS/TSTMS TO THE AREA. CONDS WILL IMPROVE AGAIN
OVERNIGHT...THOUGH A FEW ISOLATED SHOWERS AND MORE DENSE CLOUD COVER
IS EXPECTED INTO EARLY FRI. MORE SHOWER/TSTMS EXPECTED ON FRI
AFTN/EVE AS WELL.

SAT-SUN...LOCLDS AND VSBYS MAY BE AN ISSUE AS A MARINE AMS SPREADS
INLAND. THINK FLG RESTRICTIONS WL BE MORE PRONOUNCED AND PROLONGED
SAT.

MON...VFR.

&&

.MARINE...
SCA CONDITIONS NOT ANTICIPATED THROUGH THE END OF THE WORK WEEK...AS
WINDS HAVE DROPPED OFF OVERNIGHT. A FEW TSTMS POSSIBLE THIS AFTN/EVE
OVER THE WATERS...CALM AGAIN OVERNIGHT W/ ANOTHER ROUND EXPECTED ON
FRI AFTN/EVE.

ELY FLOW WL DOMINATE THIS WKND AS HIPRES BLDS DOWN THE COAST. THERE
ARE SOME HINTS THAT WNDS MAY TOUCH SCA CRIT AT TIMES...PRIMARILY IN
THE MID BAY. THAT ALL DEPENDS UPON STRENGTH OF THE SFC HIGH AND
RELATIVE LOPRES OFF THE CAROLINAS. NOT TOO CONFIDENT AT A TIGHT
P-GRAD ATTM...AND HV CAPPED WNDS BLO SCA CRIT.

&&

.TIDES/COASTAL FLOODING...
WATER LVLS ABOUT 3/4 TO 1 FT ABV ASTRO NORMS. THAT PLACED ANNAPOLIS
AT RISK FOR THE EARLY MORNING TIDE CYCLE. GAUGE READINGS CAME BACK
ARND MIDNGT...AND WE/RE TOPPING OFF JUST ABOVE MINOR CRITERIA.
DEPARTURES ELSEWHERE SHOULD NOT TRIGGER ANY ADVYS...BUT A FEW
CAUTION STAGES SHOULD BE REACHED. ONLY OTHER CONCERN MAY BE
ALEXANDRIA/SWDC...WHERE A 1.1 FT DEPARTURE WOULD TIP THE SCALE.
THINK WE MAY BE ABLE TO SNEAK THAT ONE OUT...BUT ADMITTEDLY ITS
GONNA BE CLOSE.

WE WILL HV SLY FLOW TODAY BECOMING LIGHT TNGT. FORTUNATELY THE 2ND
TIDE CYCLE OF THE DAY IS THE LOWER OF THE TWO ASTRONOMICALLY.
GUIDANCE SUGGESTING WE WL HV JUST ENOUGH OPPORTUNITY FOR A LITTLE
WATER TO OOZE OUT OF THE ESTUARY BY THEN. SO...TOMORROW MORNINGS
TIDE WL BE HIGH...BUT AM NOT ANTICIPATING REACHING CRITERIA ATTM.
HOWEVER...WE ARE WORKING ON A MARGIN OF INCHES...WHICH IS PUSHING
CURRENT SKILL LEVEL...SO ITS ADVISABLE TO CONTINUE TO MONITOR THE
SITUATION.

FLOW WL BE NLY FRI BEFORE ONSHORE /ELY/ FLOW RETURNS FOR THE WKND.
THUS...IF WATER DOESNT EVACUATE BY THE END OF THE WEEK...THERE COULD
BE MORE NUISANCE ISSUES THIS WKND.

&&

.LWX WATCHES/WARNINGS/ADVISORIES...
DC...NONE.
MD...DENSE FOG ADVISORY UNTIL 9 AM EDT THIS MORNING FOR MDZ003-004.
VA...DENSE FOG ADVISORY UNTIL 9 AM EDT THIS MORNING FOR VAZ025>031-
     036>040-042-050-051-501-502.
WV...DENSE FOG ADVISORY UNTIL 9 AM EDT THIS MORNING FOR WVZ051>053.
MARINE...NONE.

&&

$$
SYNOPSIS...HTS
NEAR TERM...GMS
SHORT TERM...GMS
LONG TERM...HTS
AVIATION...GMS/HTS
MARINE...GMS/HTS
TIDES/COASTAL FLOODING...HTS







USA.gov is the U.S. government's official web portal to all federal, state and local government web resources and services.