Area Forecast Discussion
Issued by NWS Blacksburg, VA

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FXUS61 KRNK 182342
AFDRNK

AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE BLACKSBURG VA
742 PM EDT SAT APR 18 2015

.SYNOPSIS...
HIGH PRESSURE WILL DRIFT EAST TONIGHT ALLOWING CLOUDS FROM LOW
PRESSURE OVER THE TENNESSEE VALLEY TO SPREAD ACROSS THE REGION
OVERNIGHT. RAIN...HEAVY AT TIMES...WILL FALL SUNDAY INTO SUNDAY
NIGHT BEFORE A COLD FRONT CROSSES THE REGION WITH A CHANCE OF
SHOWERS AND POTENTIALLY STRONG STORMS ON MONDAY. DRY AND COOL
CONDITIONS WILL PREVAIL FOR THE MIDDLE PART OF NEXT WEEK.

&&

.NEAR TERM /THROUGH SUNDAY/...
AS OF 712 PM EDT SATURDAY

A FEW ISOLATED SHOWERS AND THUNDERSTORMS POPPED UP THIS
AFTERNOON...MAINLY SOUTH OF HIGHWAY 460 IN THE WEST AND SOUTH OF
THE VA-NC BORDER EAST. THIS CONVECTION WILL FADE WITH LOSE OF
HEATING THIS EVENING. RAIN CHANCES WILL INCREASE OVERNIGHT AS A
STRONG AREA OF LOW PRESSURE APPROACHES THE REGION.

PREVIOUS DISCUSSION...
AS OF 300 PM EDT SATURDAY

WEAK LOW LEVEL CONVERGENCE JUST SOUTH OF OUR FORECAST AREA HAS
IGNITED A FEW SHOWERS AND ISOLATED STORMS. THE HRRR MODEL LIFTS THIS
BOUNDARY SLIGHTLY NORTH. THIS COMBINED WITH A WEAK UPSLOPE FLOW MAY
FIRE A FEW SHOWERS AND ISOLATED STORMS SOUTH OF HIGHWAY 58 THIS
AFTERNOON AND EARLY THIS EVENING...THEREFORE KEPT A SMALL CHANCE FOR
POPS IN THE FORECAST.

ALL THE MODELS ARE IN GOOD AGREEMENT ON DEVELOPING A STRONG LOW
LEVEL SOUTHEAST JET...TRANSPORTING MOISTURE INTO THE AREA OVERNIGHT.
A FEW GUSTS OVER 30KTS WILL BE POSSIBLE IN SOUTHEAST WV/CLINCH RIVER
VALLEY VA OVERNIGHT AND SUNDAY...BUT VALUES ARE BELOW GUIDANCE
CRITERIA.

PWATS INCREASE TO 2-3 STANDARD DEVIATIONS ABOVE AVERAGE BY SUNDAY
MORNING AND THROUGH MUCH OF THE DAY SUNDAY. MEANWHILE...A LARGE AREA
UVV LIFTS NORTH INTO OUR REGION TOWARD DAWN THEN EAST OVER ENTIRE
AREA ON SUNDAY. THIS IS IN ADVANCE OF A NEGATIVELY TILTED UPPER TROF
LIFTING THROUGH THE OHIO VALLEY SUNDAY WHICH ALSO GENERATES STRONG
UPPER DIFFLUENCE. AS A RESULT...EXPECT MODERATE TO HEAVY RAIN TO FALL
FOR SEVERAL HOURS...ESPECIALLY ALONG THE EAST SLOPES OF THE BLUE
RIDGE. MORE INFORMATION ABOUT THE FLOODING POTENTIAL FROM THIS
SYSTEM CAN BE FOUND IN THE HYDROLOGY SECTION. BOTTOM LINE...A FLOOD
WATCH WILL BE POSTED FOR THE NW NC MOUNTAINS/FOOTHILLS AND SOUTHERN
BLUE RIDGE OF VA. FUTURE SHIFTS MAY NEED TO EXPAND THIS NORTH AND
EAST...IF HIGHER QPF TRENDS CONTINUE.

CONTINUED TO GO BELOW GUIDANCE FOR HIGHS SUNDAY...WITH WEDGE IN
PLACE AND WIDESPREAD CLOUDS AND PRECIPITATION. IN FACT...MANY AREAS
IN THE WEDGE LIKELY SEE LITTLE...IF ANY...RECOVERY IN TEMPS.

&&

.SHORT TERM /SUNDAY NIGHT THROUGH TUESDAY NIGHT/...
AS OF 335 PM EDT SATURDAY...

HEAVIER RAINFALL ASSOCIATED WITH NORTHEASTWARD MOVING STRONGLY
NEGATIVELY TILTED UPPER TROUGH AND ASSOCIATED WARM/WEDGE FRONT
WILL MOVE INTO THE PIEDMONT THROUGH 06Z MON...THEN EAST OF THE CWA
BETWEEN 06Z AND 12Z MON. ADDITIONAL RAINFALL OF 1/2 TO 3/4 INCH
DURING THIS TIME FRAME SHOULD BE CONFINED TO THE PIEDMONT. FLOOD
WATCH ENDS AT 04Z MON...WHICH SHOULD BE GOOD TIMING. MEANWHILE...A
BROAD UPPER TROUGH CONTINUES TO DEEPEN ACROSS THE GREAT LAKES WITH
UPPER FLOW REMAINING SOUTHWEST BEHIND THE AFOREMENTIONED UPPER
TROUGH. THE NEXT UPPER SHORT WAVE WILL BE APPROACHING THE WESTERN
AREAS BY DAYBREAK MONDAY WITH A STRONG COLD FRONT MOVING INTO THE
WESTERN TN/OH VALLEY. SCATTERED SHOWERS SHOULD DEVELOP ONCE AGAIN
ACROSS THE WESTERN MOUNTAINS IN ADVANCE OF THESE FEATURES. BY
AFTERNOON...MUCH OF THE CWA...ESPECIALLY EAST OF THE BLUE RIDGE
WILL BE SOLIDLY IN THE WARM SECTOR WITH TEMPERATURES ACROSS THE
PIEDMONT NEAR 80 DEGREES AND DEWPOINTS NEAR 60. A 6-HOUR WINDOW OF
SUBSTANTIAL CAPE IN EXCESS OF 1000 J/KG...BULK 0-6KM SHEAR OF
20-30KTS...LIS TO -6 ALONG THE VA/NC BORDER...AND SIG SVR PARMS
GREATER THAN 20000 COMBINED WITH STRONG PVA AND YET ANOTHER
NEGATIVELY TILTED SHORT WAVE STRONGLY SUPPORT THE SLIGHT RISK
OUTLOOK FROM SPC EAST OF THE BLUE RIDGE MON AFTERNOON...WITH THE
MARGINAL RISK FURTHER WEST INTO THE MOUNTAINS. MAIN CONCERNS WILL
BE THUNDERSTORM WIND GUSTS AND HAIL WITH COLD AIR ALOFT ADVECTING
INTO THE REGION. A FEW MORE DISCRETE CELLS COULD DEVELOP EAST OF
THE FRONT ACROSS THE PIEDMONT IF MAXIMUM INSTABILITY/HEATING IS
REALIZED. THE SEVERE THREAT WILL END QUICKLY FROM WEST TO EAST
AFT 00Z AS THE THERMODYNAMICS WANE QUICKLY AND THE UPPER SUPPORT
SHIFTS NORTHEAST OF THE REGION. RAINFALL WILL NOT BE A CONCERN
WITH THIS SECOND EVENT AS ACTIVITY WILL BE BRIEF AND
SCATTERED...BUT A QUICK 1/2 INCH OF RAIN MAY OCCUR IN AREAS WITH
HEAVIER SHOWERS/THUNDERSTORMS. SCATTERED UPSLOPE RAIN SHOWERS
COULD PERSIST ACROSS THE MOUNTAINS INTO TUESDAY...BUT NO
SIGNIFICANT DYNAMICS ARE FORESEEN WITHIN THIS TIME FRAME.
UNSEASONABLY WARM TEMPERATURES MONDAY WILL GIVE WAY TO MUCH COOLER
CONDITIONS ACROSS THE MOUNTAINS TUESDAY WITH VARIABLE CLOUDS AND A
FEW SHOWERS...BUT DOWNSLOPE PIEDMONT AREAS SHOULD STILL BE CLOSE
TO 70 EVEN TUESDAY.

&&

.LONG TERM /WEDNESDAY THROUGH SATURDAY/...
AS OF 350 PM EDT SATURDAY...

MODELS IN BETTER AGREEMENT TODAY ON A DEEP AND BROAD UPPER LOW
CENTERED ACROSS THE EASTERN GREAT LAKES DOMINATING THE WEATHER
THROUGH MOST OF THE REMAINDER OF THE WORK WEEK. THIS WILL LEAVE
UNSETTLED CONDITIONS ACROSS THE MOUNTAINS...SOMEWHAT LESS SO
ACROSS THE PIEDMONT. THE ECMWF HAS COME INTO BETTER AGREEMENT WITH
THE GFS NOW ON A DISTURBANCE ROTATING THROUGH THE BOTTOM OF THE
UPPER TROUGH WEDNESDAY...BRINGING A BETTER THREAT OF SHOWERS TO
THE REGION. THUS...HAVE INCREASED POPS IN ACCORDANCE WITH THE
BETTER MODEL AGREEMENT AND SURROUNDING OFFICES. HAVE NOT
INTRODUCED THUNDER AT THIS POINT...BUT COULD NOT TOTALLY RULE OUT
SOME THUNDER BECAUSE OF RATHER COLD AIR ALOFT. FOLLOWING THIS
DISTURBANCE COMES THE BONAFIDE COLDER AIR WHICH PROMISES TO PUSH
850MB TEMPS DOWN TO 0C OR BELOW FOR MUCH OF THE REMAINDER OF THE
WEEK ACROSS THE NORTHERN AREAS. THE UPPER TROUGH GRADUALLY SHIFTS
MORE TOWARD THE MID-ATLANTIC LATE IN THE PERIOD...STRENGTHENING
THE NORTHWEST FLOW ACROSS THE REGION...AND ALLOWING 850MB TEMPS TO
BOTTOM OUT ACROSS THE NORTH AROUND -5C BY SATURDAY. THIS PATTERN
WILL CONTINUE TO RESULT IN A SIGNIFICANT TEMPERATURE DIFFERENCE
FROM THE WESTERN MOUNTAINS WITH CLOUDS AND VARIABLE
CLOUDS/SCATTERED SHOWERS AND MORE SUNNY/DOWNSLOPE NORTHWEST FLOW
ACROSS THE PIEDMONT. OVERALL...TEMPERATURES SHOULD AVERAGE BELOW
NORMAL THROUGH THE PERIOD...PERHAPS MUCH BELOW IN THE WEST. MEXMOS
SHOWS SEVERAL DAYS WITH HIGHS ONLY IN THE 50S/60S WEST AND LOWS IN
THE UPPER 30S TO LOWER 40S. WIND AND CLOUDS SHOULD PREVENT ANY
FROST/FREEZE CONCERN AT THIS POINT.

&&

.AVIATION /00Z SUNDAY THROUGH THURSDAY/...
AS OF 735 PM EDT SATURDAY...

ISOLATED SHOWERS ALONG THE BLUE RIDGE SHOULD DISSIPATE SHORTLY
WITH LOSS OF HEATING.

MODELS CONTINUE TO BE CONSISTENT WITH CLOUDS AND THEN RAIN WILL
SPREAD INTO THE AREA FROM SOUTHWEST TO NORTHEAST TONIGHT. EXPECT
CEILINGS TO LOWER TO MVFR ON THE 06-09Z TIME RANGE.

A STRONG SOUTHEAST LOW LEVEL JET WILL DEVELOP JUST AFTER MIDNIGHT
WHICH MAY BRING WIND GUSTS TO 30 KTS AT KLBF EARLY SUNDAY AND
CONTINUE THROUGH 18Z. WITH A WEDGE OF HIGH PRESSURE IN PLACE ALONG
AND EAST OF THE BLUE RIDGE...THESE STRONG WINDS ARE NOT EXPECTED
TO REACH THE SURFACE AND KBCB AND KLWB...BUT WILL CREATE
POTENTIALLY HAZARDOUS LOW LEVEL WIND SHEAR CONDITIONS. KROA WILL
BE NEED TO BE CAREFULLY MONITORED TO POSSIBLE INCLUSION OF LL WIND
SHEAR.

CIGS WILL CONTINUE TO DROP AND RAIN WILL BECOME WIDESPREAD DURING
THE DAY SUNDAY...AS A STRONG UPSLOPE FLOW TRANSPORTS ABUNDANT
MOISTURE INTO THE REGION NORTH OF A NEARLY STATIONARY FRONT.
WIDESPREAD IFR CONDITIONS ARE FORECAST IN THE MOUNTAINS AND
FOOTHILLS...WITH MVFR CIGS AT KLYH AND KDAN LIKELY DETERIORATING
AFTER 18Z SUNDAY WHEN THE HEAVIEST PRECIPITATION ARRIVES.

EXTENDED AVIATION...

A COLD FRONT APPROACHES LATER MONDAY WITH SOME STORMS POSSIBLE
AHEAD OF AND ALONG IT...ESPECIALLY EAST OF THE BLUE RIDGE. THIS
IS LIKELY TO KEEP PERIODS OF MVFR AND BRIEF IFR IN STORMS UNTIL
THE FRONT CLEARS THE REGION MONDAY EVENING

VFR RETURNS LATER MONDAY NIGHT AND TUESDAY WITH HIGH
PRESSURE BUILDING IN BEHIND THE FRONT WITH A DRIER AIRMASS.

THEN THE NEXT CHANCE OF MVFR CONDITIONS AND PRECIPITATION WILL BE
IN THE MOUNTAINS ON WEDNESDAY.

.HYDROLOGY...
AS OF 330 PM EDT SATURDAY...

ANTECEDENT CONDITIONS ARE STILL MOIST ACROSS THE NW NC
MOUNTAINS/FOOTHILLS INTO THE SOUTHERN BLUE RIDGE OF VA WITH THE
PERCENT OF NORMAL PRECIP OVER THE PAST WEEK EXCEEDING OVER 150%.
THIS ALSO IS SHOWN IN RIVER STAGE LEVELS ACROSS THIS REGION FROM
THE USGS WITH STREAM LEVELS IN MANY LOCATIONS RUNNING GREATER THAN
THE 75TH PERCENTILE. WITH 6 HOUR FLASH FLOOD GUIDANCE VALUES BELOW
3 INCHES IN THE REGION...AND FORECAST PW VALUES 2-3 STANDARD
DEVIATIONS ABOVE NORMAL...EXPECT FLOODING OF CREEKS AND STREAMS.
WHILE THE SREF MODEL ENSEMBLE RIVER FORECAST ONLY IS FORECASTING A
SMALL CHANCE FOR RIVERS TO REACH ACTION STAGE...WOULD NOT BE
SURPRISED TO SEE A FEW SITES REACH MINOR FLOODING CONSIDERING THE
MODEL TRENDS OF INCREASING QPF WITH EACH RUN.

&&

.RNK WATCHES/WARNINGS/ADVISORIES...
VA...FLOOD WATCH FROM 4 AM EDT SUNDAY THROUGH SUNDAY EVENING FOR
     VAZ015>017-032.
NC...FLOOD WATCH FROM 4 AM EDT SUNDAY THROUGH SUNDAY EVENING FOR
     NCZ001>003-018>020.
WV...NONE.

&&

$$
SYNOPSIS...PH
NEAR TERM...PH/RCS
SHORT TERM...RAB
LONG TERM...RAB
AVIATION...PC/PH
HYDROLOGY...PH


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