Area Forecast Discussion
Issued by NWS Tucson, AZ

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FXUS65 KTWC 171459
AFDTWC

AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE TUCSON AZ
759 AM MST WED DEC 17 2014

.SYNOPSIS...A TROUGH OF LOW PRESSURE WILL BRING VALLEY RAIN...
MOUNTAIN SNOW...AND BELOW AVERAGE DAYTIME TEMPERATURES INTO
THURSDAY. DRY CONDITIONS WITH A GRADUAL WARMING TREND WILL THEN
PREVAIL FRIDAY INTO EARLY NEXT WEEK.

&&

.DISCUSSION...FAIRLY BROAD AREA OF SHOWERS MOVING THROUGH THE
EASTERN HALF OF THE FORECAST AREA THIS MORNING. GENERALLY
ACCUMULATIONS WITH THESE SHOWERS HAVE BEEN LIGHT. THERE WERE A FEW
LIGHTNING STRIKES WITH THESE SHOWERS AS THE MAIN VORT
LOBE/SHORTWAVE ENERGY MOVED THROUGH...THOUGH DON`T EXPECT MUCH
MORE WITH THIS EVENT.

THERE IS A SECONDARY VORT LOBE MOVING THROUGH SOUTHERN CALIFORNIA
THIS MORNING THAT HAS SOME LIGHTNING WITH IT EARLIER. THIS LOBE
WILL LIKELY PUSH INTO THE AREA THIS AFTERNOON WHICH WILL BRING
ANOTHER ENHANCED AREA OF SHOWERS. BETWEEN THE FIRST AND SECOND
VORT LOBE THERE WILL BE SOME SHOWER ACTIVITY THOUGH IT WILL NOT
LIKELY BE AS WIDESPREAD.

WILL LIKELY SEE A BREAK IN PRECIPITATION DURING THE EARLY MORNING
HOURS AS THE SECOND VORT LOBE EXITS...THOUGH ENOUGH MOISTURE
ESPECIALLY ACROSS THE NORTHERN ZONE TO KEEP SOME MENTION OF
PRECIPITATION IN THE FORECAST. THEN SLIGHTLY BEFORE DAYBREAK
TOMORROW SHOULD SEE A THIRD VORT LOBE PUSH THROUGH BRINGING
ANOTHER ROUND OF PRECIPITATION.

A FOURTH AND MUCH WEAKER VORT LOBE WILL SLIDE DOWN THE BACKSIDE
OF THE UPPER LOW LATE THURSDAY...WHICH WILL LIKELY JUST HELP CLEAR
THINGS OUT. FROM THEN ON EXPECT HIGH PRESSURE ALOFT TO BUILD OVER
THE SOUTHERN CALIFORNIA COAST. THIS WILL KEEP THINGS DRY
THROUGHOUT THE REMAINDER OF THE FORECAST. THE ONLY CAVEAT TO THAT
IS SOME GUIDANCE IS SHOWING SOME HINTS OF ANOTHER SYSTEM NEAR
CHRISTMAS. THIS REMAINS OUTSIDE OF THE FORECAST PERIOD AND
CONFIDENCE REMAINS VERY LOW IN ITS OCCURRENCE.

SNOW FALL IN THE MOUNTAINS DURING THE CURRENT EVENT WILL REMAIN
BELOW ADVISORY CRITERIA...WITH MOST ACCUMULATIONS ABOVE 7000 FT.
SNOW LEVELS WILL GENERALLY REMAIN THE SAME THROUGH THE
EVENT...THOUGH COULD SOME LOWERING THURSDAY BEFORE THE MOISTURE
EXITS.


&&

.AVIATION...VALID THRU 18/12Z. AREAS OF CIGS BKN-OVC 4-6K FT AGL
WITH MULTIPLE BKN-OVC LAYERS ABOVE. EXPECT LOCAL MVFR CIGS THRU
THE PERIOD GENERALLY 2-3K FT. SCATTERED -SHRA/MT -SHSN ACROSS THE
AREA WILL CONTINUE OFF AND ON THRU 18/03Z THEN BECOME MORE
ISOLATED. EXPECT SLY SFC WIND 5-10 KTS THIS MORNING EXCEPT LOCAL
SW WIND TO 15 KTS FAR SRN AREAS. WIND BECOMING SW 8-15 KTS MOST
AREAS AFT 17/17Z THEN EASING TO LIGHT AND VARIABLE AFT 18/03Z.
AVIATION DISCUSSION NOT UPDATED FOR TAF AMENDMENTS.

&&

.FIRE WEATHER...A COUPLE OF STORM SYSTEMS WILL BRING A FEW PERIODS
OF VALLEY RAIN AND MOUNTAIN SNOW TODAY AND AGAIN THURSDAY WITH
SOMEWHAT OF A BREAK IN THE PRECIPITATION TONIGHT. SNOW LEVELS WILL
BE AS LOW AS 6000 FT WITH ACCUMULATING SNOWS OCCURRING GENERALLY
ABOVE 7000 FT WITH SEVERAL INCHES POSSIBLE. BREEZY SOUTHERLY WINDS
ARE EXPECTED LATE THIS MORNING UNTIL THE EARLY EVENING HOURS WITH
LIGHTER WESTERLY WINDS THURSDAY. THEREAFTER...DRY WEATHER IS
EXPECTED THROUGH THE COMING WEEKEND WITH PLENTY OF PASSING HIGHER
CLOUDS AND FAIRLY LIGHT NORTH TO NORTHWEST WINDS.

&&

.PREV DISCUSSION... /ISSUED 515 AM MST WED DEC 17 2014/

UPDATE...17/12Z KTWC SOUNDING TOTAL PRECIP WATER VALUE IS 0.68
INCH. HAVE INCREASED QPF/S TODAY TO GENERALLY RANGE FROM ONE TENTH
OF AN INCH TO ONE THIRD OF AN INCH IN THE VALLEYS...AND LIQUID
AMOUNTS TO NEAR ONE HALF OF AN INCH ACROSS THE HIGHEST PEAKS. SNOW
ACCUMS TODAY WERE INCREASED TO GENERALLY 2-4 INCHES ABOVE 7000
FEET...WITH AROUND 5 INCHES OF SNOW ACROSS THE HIGHEST PEAKS.
THESE ACCUMS ARE JUST BELOW SNOW ADVISORY CRITERIA FOR TODAY.

DISCUSSION...DEEP UPPER TROUGH CONTINUES OVER THE WRN CONUS EARLY
THIS MORNING...AND A SERIES OF VORTICITY MAXIMA WILL MOVE THRU THE
BASE OF THIS TROUGH INTO THUR. THE FIRST SHORTWAVE TROUGH IS
PRESENTLY MOVING NEWD ACROSS SE AZ...AND HAS PRODUCED SCATTERED TO
NUMEROUS SHOWERS. RAIN GAUGE AMOUNTS HAVE ONLY MOSTLY RECORDED A FEW
HUNDREDTHS OF AN INCH THRU 315 AM MST...ALTHOUGH A FEW LOCALES ACROSS
FAR WRN PIMA COUNTY HAVE MEASURED UP TO NEARLY ONE HALF OF AN INCH.
SNOW LEVELS HAVE FALLEN TO AROUND 6500 FEET GIVEN TEMPS FROM VARIOUS
RAWS SITES. A FEW LIGHTNING STRIKES HAVE BEEN DETECTED AS FAR NORTH
AS EXTREME SOUTH CENTRAL PIMA COUNTY THRU 315 AM MST.

17/00Z NAM/GFS/ECMWF DEPICTED A CONSIDERABLE REDUCTION IN QPF/S INTO
THUR VERSUS SOLUTIONS FROM 24 HOURS OR MORE AGO. BASED ON THESE
VARIOUS NWP MODELS...HAVE CONTINUED WITH SCATTERED TO NUMEROUS
SHOWERS TODAY. HAVE ALSO INTRODUCED A SLIGHT CHANCE OF THUNDERSTORMS
GIVEN THAT MARGINAL INSTABILITY SHOULD PREVAIL AS COLD AIR ALOFT
MOVES ACROSS THE AREA. WOULD NOT BE SURPRISED TO RECEIVE REPORTS OF
SMALL HAIL OCCURRING LATER TODAY. SNOW LEVELS THIS AFTERNOON ARE
PROJECTED TO RANGE FROM AROUND 6000 FEET ACROSS THE WHITE MOUNTAINS
TO 6500-7000 FEET FROM TUCSON SWD TO THE INTERNATIONAL BORDER.
LIQUID AMOUNTS TODAY ARE EXPECTED TO GENERALLY RANGE FROM A FEW
HUNDREDTHS OF AN INCH TO AROUND ONE QUARTER OF AN INCH. SNOW
ACCUMULATIONS TODAY SHOULD THEN RANGE FROM 1-3 INCHES ABOVE 7000
FEET FROM THE CATALINA/RINCON MOUNTAINS NEAR TUCSON NEWD TO THE
WHITE MOUNTAINS.

17/00Z NAM/GFS DEPICTED CONSIDERABLY LESSER COVERAGE OF PRECIP TO
OCCUR TONIGHT. HOWEVER...THE 17/00Z ECMWF WAS MORE ROBUST WITH
PRECIP CHANCES TONIGHT. THE INHERITED POPS WERE REDUCED MARKEDLY...
BUT STILL REFLECT ISOLATED TO SCATTERED COVERAGE OF SHOWERS TONIGHT.
SNOW LEVELS AROUND DAYBREAK THUR SHOULD RANGE FROM AROUND 5000 FEET
IN THE WHITE MOUNTAINS TO NEAR 6000 FEET ADJACENT THE INTERNATIONAL
BORDER. THE HIGHEST SNOW ACCUMS TONIGHT SHOULD ONLY BE AROUND 2
INCHES ACROSS THE HIGHEST PEAKS.

17/00Z GFS WAS CONSIDERABLY DRIER VERSUS THE 17/00Z NAM/ECMWF THUR.
A DEEPER SHORTWAVE TROUGH IS PROGGED TO MOVE EWD ACROSS SE AZ/SONORA
MEXICO THUR. ALTHOUGH THE NAM/ECMWF WERE WETTER VERSUS THE GFS...THE
NAM/ECMWF DEPICTED LIGHTER QPF/S THUR VERSUS TODAY. THUS...THE
EXPECTATION FOR THUR IS FOR GENERALLY LIGHTER RAIN/SNOW AMOUNTS TO
OCCUR VERSUS TODAY. HOWEVER...THE EXPECTED COVERAGE OF PRECIP JUSTIFIED
CHANCE CATEGORY POPS THUR FROM TUCSON EWD...AND A SLIGHT CHANCE OF
SHOWERS ACROSS WRN PIMA COUNTY. A SLIGHT CHANCE OF VALLEY RAIN/MOUNTAIN
SNOW SHOWERS CONTINUES THUR EVENING ACROSS ERN SECTIONS FOLLOWED BY
DRY CONDITIONS AREA-WIDE LATE THUR NIGHT.

LIQUID AMOUNTS ON THUR ARE FORECAST TO BE GENERALLY A FEW HUNDREDTHS
OF AN INCH TO PERHAPS AS MUCH AS TWO TENTHS OF AN INCH. SNOWFALL
ACCUMS THUR WILL THEN BE FROM 1-2 INCHES ABOVE 7000 FEET. GIVEN ALL
OF THE AFOREMENTIONED FORECAST SNOWFALL ACCUMS...AND HAVING OPTED TO
TREAT THE TWO MAIN PRECIP EVENTS (I.E. "TODAY" AND "THURSDAY") AS
SEPARATE EVENTS...NO ISSUANCE OF A WINTER WEATHER ADVISORY FOR SNOW
WILL OCCUR AT THIS TIME.

NWLY FLOW ALOFT WILL RESULT IN DRY CONDITIONS FRI-TUE IN RESPONSE TO
HIGH PRESSURE ALOFT THAT IS PROGGED TO AMPLIFY WEST OF THE SRN
CALIFORNIA COAST. HIGH TEMPS TODAY AND THUR WILL AVERAGE ABOUT 3-10
DEGS F BELOW NORMAL AREA-WIDE...THEN A GRADUAL WARMING TREND WILL
OCCUR FRI-TUE.

&&

.TWC WATCHES/WARNINGS/ADVISORIES...NONE.

&&

$$

DISCUSSION...MEADOWS

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