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Area Forecast Discussion

183
FXUS64 KHUN 181040
AFDHUN

AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE HUNTSVILLE AL
540 AM CDT SAT APR 18 2015

.DISCUSSION...
LATEST WATER VAPOR SATELLITE ANIMATION SUGGESTS THAT A WEAK 500-MB
SHORTWAVE TROUGH WHICH BROUGHT RAINFALL TO THE REGION YESTERDAY
CONTINUES TO SHIFT EASTWARD INTO THE CAROLINAS THIS MORNING. IN THE
WAKE OF THIS FEATURE...A SHORT WAVELENGTH MID-LEVEL RIDGE WILL BUILD
QUICKLY EASTWARD OVER THE REGION THIS MORNING DOWNSTREAM FROM LARGE
UPPER LOW SITUATED ACROSS CENTRAL COLORADO. THIS WILL MAINTAIN A
TRANQUIL WEATHER PATTERN THROUGH EARLY AFTERNOON...WITH ONLY MAJOR
CONCERN BEING AREAS OF MORNING FOG -- SOME OF WHICH COULD BE LOCALLY
DENSE ACROSS PORTIONS OF THE TN VALLEY. FOG IS EXPECTED TO LIFT AND
SCATTER INTO A BROKEN DECK OF STRATOCUMULUS CLOUDS BY 15Z.
MEANWHILE...AS THE RIDGE AXIS ALOFT QUICKLY TRANSLATES EASTWARD THIS
AFTERNOON...A RETURN TO STRONG SSW FLOW ALOFT WILL OCCUR WITH THE
FIRST IN A SERIES OF WEAK DISTURBANCES SHIFTING NORTHEASTWARD LATE IN
THE PERIOD. INCREASING LIFT ASSOCIATED WITH THIS FEATURE MAY INITIATE
DEVELOPMENT OF ISOLATED CONVECTION ALONG REMNANTS OF BACKDOOR FRONT
WHICH PUSHED ACROSS THE REGION A COUPLE OF DAYS AGO. THIS ACTIVITY
WILL PRIMARILY IMPACT NORTHERN/EASTERN PORTIONS OF THE FORECAST AREA.

BY TONIGHT...THE COLORADO UPPER LOW IS FORECAST TO SPREAD EASTWARD
INTO THE CENTRAL PLAINS AND OPEN INTO A WAVE AS IT BEGINS TO
INTERACT WITH A STRONGER NORTHERN STREAM TROUGH OVER SOUTH CENTRAL
CANADA. AS THIS OCCURS...SHORTWAVE VORTICITY MAXIMA CURRENTLY MOVING
ONSHORE ALONG THE CENTRAL PACIFIC COAST OF MEXICO WILL EJECT RAPIDLY
NORTHEASTWARD ACROSS THE LOWER MISSISSIPPI VALLEY AND INTO OUR REGION
BY 12Z SUNDAY. THIS WILL LEAD TO A RAPID INCREASE IN DEEP-LAYER
FORCING FOR ASCENT BETWEEN 00-06Z...WITH AREA OF WIDESPREAD RAIN AND
EMBEDDED THUNDERSTORMS PROGGED TO LIFT NORTHEASTWARD -- ENCOMPASSING
NEARLY ENTIRE FORECAST AREA BY 06Z. ASSOCIATED SURFACE CYCLONE WILL
GRADUALLY DEEPEN AS IT LIFTS NORTHEASTWARD INTO WESTERN TN BY 12Z
SUNDAY...WITH SYNOPTIC WARM FRONT FORECAST TO LIFT NORTHWARD BETWEEN
06-12Z ACCOMPANIED BY INCREASING SOUTHERLY FLOW AND DEWPOINTS RISING
INTO THE MID 60S. SUBTLE STEEPENING OF MID-LEVEL LAPSE RATES WILL
SUPPORT A MORE WIDESPREAD CONVECTIVE SHOWER EVENT DURING THIS
PERIOD...ALONG WITH AN INCREASING THREAT FOR STORMS. AS SSWLY LOW-
LEVEL JET INCREASES TO 30-40 KNOTS IN WAKE OF WARM FRONT...0-3 KM SRH
VALUES WILL INCREASE INTO THE 200-300 M2/S2 RANGE WHICH RAISES THE
THREAT FOR LOW-LEVEL MESOCYCLONE DEVELOPMENT WITH ANY DEEPER
CONVECTIVE CELLS. THUS...ALTHOUGH THREAT FOR LARGE HAIL IS LOW...A
BRIEF TORNADO IS CERTAINLY POSSIBLE.

GLOBAL MODELS ARE IN GOOD AGREEMENT THAT REMNANTS OF EARLY MORNING
PRECIPITATION WILL BE QUICKLY LIFTING NORTHEASTWARD AND ENDING FOR
LOCATIONS ALONG AND EAST OF INTERSTATE 65 BETWEEN 12-15Z.
MEANWHILE...DEEPENING SURFACE LOW WILL CONTINUE TO LIFT NORTHEASTWARD
INTO THE SOUTHERN LAKE MICHIGAN VICINITY BY 00Z MONDAY...WITH THE
LOCAL AREA EXPECTED TO REMAIN IN WARM SECTOR AIRMASS FOR THE ENTIRE
DAY. AS CLOUDS CLEAR IN WAKE OF MORNING PRECIP...VERY UNSTABLE
CONDITIONS WILL DEVELOP WITH DEWPOINTS REMAINING IN THE MID 60S AND
TEMPS RISING INTO THE UPPER 70S/LOWER 80S AIDED BY A GUSTY SW WIND.
POINT FORECAST SOUNDINGS SUGGEST THAT AN EML WILL BE ADVECTED ACROSS
THE REGION WITH STEEP 700-500 MB LAPSE RATES SUPPORTING 1000-2000
J/KG CAPE IN THE AFTERNOON. THAT SAID -- WITH THE EXCEPTION OF A WEAK
MID-LEVEL WAVE PASSING AROUND MIDDAY -- DEEP LAYER FORCING FOR
ASCENT WILL BE RATHER WEAK AND THUS EXPECT WARM SECTOR CONVECTION TO
REMAIN WIDELY SCATTERED. ANY STORMS THAT DO MANAGE TO DEVELOP WILL
ALMOST CERTAINLY BE SEVERE AND CAPABLE OF PRODUCING VERY LARGE HAIL
AND DAMAGING WINDS.

AS REMNANTS OF CENTRAL PLAINS MID-LEVEL TROUGH SPREAD NORTHEASTWARD
INTO THE OHIO VALLEY SUNDAY NIGHT...A SECONDARY WAVE OF LOW
PRESSURE WILL FORM ALONG TRAILING COLD FRONT OF INITIAL SYSTEM.
PREFRONTAL SURFACE TROUGH ASSOCIATED WITH THE DEEPENING LOW WILL
SWEEP EASTWARD BETWEEN 06-12Z MONDAY PROVIDING ANOTHER ROUND OF
CONVECTION. MID- LEVEL FORCING AND ASSOCIATED LOW-LEVEL CONVERGENCE
WILL BE RATHER WEAK...WITH THIS CONVECTION EXPECTED TO REMAIN
SCATTERED AS WELL. HOWEVER...DEEP-LAYER SHEAR WILL INCREASE INTO THE
40-45 KNOT RANGE SUPPORTING A THREAT FOR SUPERCELLS. AIRMASS WILL
REMAIN QUITE UNSTABLE...AND THIS ACTIVITY WILL ALSO POSE A RISK OF
DAMAGING WINDS...LARGE HAIL AND PERHAPS A BRIEF TORNADO. CLOUDS AND
LINGERING SHOWERS WILL BE POSSIBLE THROUGH THE DAY ON MONDAY UNTIL
TRUE COLD FRONT CLEARS REGION LATE IN THE AFTERNOON.

IN THE EXTENDED PERIOD...BENIGN ZONAL FLOW WILL PERSIST FROM MONDAY
NIGHT-TUESDAY NIGHT TO THE SOUTH OF A STRONG UPPER LOW OVER THE
NORTHERN GREAT LAKES/SOUTHERN ONTARIO. WINDS ALOFT WILL VEER SLIGHTLY
TO NORTHWEST ON WEDNESDAY/THURSDAY AS A MID-LEVEL RIDGE EXTENDING
FROM THE SOUTHERN PLAINS NORTHWARD INTO THE CANADIAN ROCKIES SHIFTS
EASTWARD IN RESPONSE TO TROUGH PUSHING ONTO THE PACIFIC COAST.
ALTHOUGH A COUPLE OF WEAK MID-LEVEL SHEAR ZONES WILL SPREAD
SOUTHEASTWARD IN THIS REGIME...COOLER AND MORE STABLE CONDITIONS WILL
PRECLUDE THUNDERSTORMS UNTIL INSTABILITY INCREASES THURSDAY.
GFS/ECMWF BOTH AGREE THAT THE PACIFIC TROUGH WILL EMERGE INTO THE
PLAINS BY END OF WEEK...WITH WARM FRONT LIFTING NORTHWARD INTO REGION
AND BRINGING AN INCREASE IN SHOWER/THUNDERSTORM ACTIVITY BY FRIDAY.

70/DD

&&

.AVIATION...
/ISSUED 1235 AM CDT SAT APR 18 2015/
FOR 06Z TAFS...ALREADY DEALING WITH FOG ISSUES AT KHSV AND KMSL AS OF
05Z. TAFS FOR BOTH SITES WILL BE PESSIMISTIC INDICATING TEMPO 1/4SM
AND VV001 WITH PREVAILING HIGHER CATEGORIES. HOWEVER...INCOMING
CIRRUS FROM STORMS ALONG THE NW GULF COAST COULD KEEP THE WORST CASE
SCENARIO FROM OCCURRING. CONDITIONS SHOULD SLOWLY IMPROVE AFTER
DAYBREAK...WITH VFR WEATHER IN THE AFTERNOON. A SYSTEM MOVING IN FROM
THE WEST WILL BRING MORE MVFR CIGS AND -SHRA SAT EVENING. LATE IN THE
TAF...MVFR AND IFR WX SHOULD INCREASE. THERE COULD BE CONVECTION LATE
IN PERIOD...BUT HELD OFF DUE TO UNCERTAINLY THIS ISSUANCE.

BCC/RSB

&&

.PRELIMINARY POINT TEMPS/POPS...
HUNTSVILLE    76  64  80  59 /  20 100  50  50
SHOALS        77  63  81  58 /  30 100  30  40
VINEMONT      75  63  80  59 /  30 100  40  40
FAYETTEVILLE  75  62  78  58 /  20 100  60  60
ALBERTVILLE   74  62  78  58 /  30 100  60  40
FORT PAYNE    74  62  77  59 /  20 100  80  50

&&

.HUN WATCHES/WARNINGS/ADVISORIES...
AL...NONE.
TN...NONE.

&&

$$

FOR MORE INFORMATION PLEASE VISIT OUR WEBSITE
AT WEATHER.GOV/HUNTSVILLE.



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