Area Forecast Discussion
Issued by NWS Phoenix, AZ

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FXUS65 KPSR 020330
AFDPSR

AREA FORECAST DISCUSSION ...CORRECTION
NATIONAL WEATHER SERVICE PHOENIX AZ
830 PM MST SUN MAR 1 2015

...POTENTIAL FOR HEAVY RAIN AFFECTING THE SOUTHWEST AND SOUTH
CENTRAL AZ DESERTS THROUGH LATE MONDAY...

&&

.SYNOPSIS...
WIDESPREAD PRECIPITATION WILL EXPAND FROM SOUTHEAST CALIFORNIA INTO
ARIZONA TONIGHT AND SLOWLY SHIFT EASTWARD DURING THE DAY MONDAY.
PERIODS OF MODERATE TO HEAVY RAIN ARE ANTICIPATED...ESPECIALLY OVER
HIGHER TERRAIN AND FOOTHILL LOCATIONS. THERE WILL ALSO BE A SLIGHT
CHANCE OF THUNDERSTORMS DURING THE DAY MONDAY...MAINLY OVER PORTIONS
OF ARIZONA. COOLER TEMPERATURES WILL ALSO ACCOMPANY THIS SYSTEM WITH
BELOW NORMAL READINGS EXPECTED MONDAY THROUGH MID WEEK. A RETURN TO
ABOVE NORMAL TEMPERATURES IS LIKELY BY LATE WEEK.

&&

.DISCUSSION...
THE COLD UPPER TROF THAT HAD BEEN DEVELOPING SLOWLY SOUTH ALONG THE
CENTRAL AND SOUTHERN CA COAST THE PAST 36 HOURS...AND HAS BEEN
SLOWLY CIRCULATING APPRECIABLE AMOUNTS OF LOW LEVEL MOISTURE INTO
THE REGION...HAS BEGUN TO KICK INLAND THIS EVENING. SAT IMAGERY
SHOWED A WELL DEFINED KICKER TROF MOVING RAPIDLY SOUTH THROUGH THE
PACIFIC NORTHWEST UNDER NORTHERLY FLOW ALOFT.

AS A RESULT...REGIONAL UPPER AIR DATA SHOWED A PROFOUNDLY SHARP
BAROCLINIC ZONE ACROSS SOUTHERN CA/SOUTHERN NEVADA THAT HAS BACKED
AND INCREASED THE WINDS ALOFT SINCE MORNING. 700 MB WINDS ALONG THE
COLORADO RIVER WERE NEARLY SOUTH AT 40 KNOTS WHILE 300 MB WINDS WERE
WERE SOUTHWEST 100-120 KNOTS AND DIFLUENT. ALSO...MORE BOUNDARY
LAYER MOISTURE HAS TRANSPORTED NORTH ALONG THE FRONTAL ZONE AND
CURRENTLY FOCUSED IN SOUTHEAST CA/COLORADO RIVER WHERE FAST MOVING
BANDS OF PRECIP HAVE INCREASED THE PAST FEW HOURS.

THEREFORE AS THE COLD DYNAMICAL SYSTEM MOVES INLAND FROM OFFSHORE
SOUTHERN CA/NORTHERN BAJA TONIGHT...AN UPSWING IN PRECIP IS
EXPECTED. PRECIP WILL FOCUS ALONG THE COLORADO RIVER BY MIDNIGHT...
MOVING INTO SOUTHWEST AZ BY 12Z MON...THEN INTO CENTRAL AZ INCLUDING
PHOENIX BY 18Z MON. SINCE SOUTHERLY LOW LEVEL WINDS ARE FAIRLY
STRONG...ENHANCED UPSLOPE PRECIP WILL BE A BIG FACTOR IN THE
POTENTIAL FOR HEAVY RAIN AND RUNOFF. THE FLASH FLOOD WATCH THROUGH
MONDAY...FOCUSED IN THE UPSLOPE AREAS OF SOUTHWEST AND SOUTH CENTRAL
AZ WHERE MOUNTAIN WASHES DRAIN SOUTH INTO THE LOWER DESERTS LOOKS
GOOD FOR NOW.  A QUICK LOOK AT SOME OF THE OFFICE WARM COLLISION
COALESCENCE RAIN STUDIES INDICATE THE POTENTIAL FOR 0.50-0.60 INCHES
ON THE LOWER CENTRAL DESERTS TO NEARLY 2 INCHES IN THE MOUNTAINS AND
FOOTHILLS NORTHWEST THROUGH EAST OF PHOENIX MONDAY.

CURRENT FORECASTS LOOK IN GOOD SHAPE WITH HIGH PRECIP PROBABILITIES
AND THE POTENTIAL FOR HEAVY RAIN. NO UPDATES PLANNED. PREVIOUS
DISCUSSION BELOW STILL APPLIES.



.PREVIOUS DISCUSSION...302 PM MST...

REST OF TODAY THROUGH WEDNESDAY...
SHOWERS HAVE MOVED OVER OUR SOUTHEAST CALIFORNIA ZONES TODAY.
ISOLATED SHOWERS HAVE STARTED TO DEVELOP ELSEWHERE INCLUDING THE
PHOENIX AREA. MADE SOME EARLIER CHANGES TO THE FORECAST TO EXPAND
THE AREAL FLOOD WATCH TO SOUTHEAST CALIFORNIA. THE OTHER MAIN CHANGE
INCLUDED ANOTHER TIMING ADJUSTMENT FOR SOUTH-CENTRAL
ARIZONA...INCLUDING THE PHOENIX AREA...FOR A LATER START AND END. IN
SHORT...ANTICIPATE A SLOW RUNOFF RESPONSE TO THE RAINFALL BUT EXPECT
THERE WILL BE WASHES WITH SUFFICIENT FLOW TO CAUSE DANGEROUS
CONDITIONS AT SOME OF THE UNBRIDGED CROSSINGS. THERE WILL BE SOME
HIGHWAY DIPS AND OTHER LOW LYING AREAS THAT WILL SEE SOME INUNDATION
AS WELL. THE MAIN PRECIP MECHANISMS WITHIN THE LOW PRESSURE SYSTEM
ARE THE FRONTAL ZONE...CURRENTLY EXTENDING SOUTHWESTWARD FROM THE
MOHAVE DESERT...AND DYNAMICAL FORCING FROM SOME DIFFLUENCE RELATED
UPPER LEVEL DIVERGENCE...MAINLY NORTHWEST ARIZONA...AS WELL AS QG
FORCING FROM THE VORTICITY AND WARM AIR ADVECTION. THERE WILL ALSO
BE SOME INFLUENCE FROM THE RIGHT ENTRANCE REGION OF THE JET FOR
TOMORROW. THE FRONT IS ANTICIPATED TO MOVE THROUGH THE PHOENIX AREA
LATE MORNING/MIDDAY WITH PRECIP TRENDING DOWN FROM WEST TO EAST OVER
THE LOWER DESERTS OF ARIZONA IN MONDAY AFTERNOON...CONTINUING HIGHER
TERRAIN. THE MODELS ARE IN GENERALLY GOOD AGREEMENT ON THE TIMING
AND PLACEMENT OF THESE FEATURES THOUGH THE GFS IS A BIT MORE
GENEROUS WITH THE QG FORCING THAN NAM WHILE ECMWF IS A BIT SLOWER
WITH THE ONSET AND DEPARTURE OF PRECIP THAN NAM AND GFS. SEE THE
FLOOD WATCH FOR DETAILS ON THE TIMING AND PRECIP AMOUNTS. THE
UPSTREAM SYSTEM THAT WILL BE HELPING TO MOVE THIS MAIN ONE ALONG IS
CURRENTLY CENTERED NEAR THE ALASKA/CANADA BORDER. IT WILL RAPIDLY
MOVE DOWN THE COAST MONDAY AND THEN INTO NORTHWEST MEXICO TUESDAY.
MEANWHILE ANOTHER SHORT WAVE TRACKS THROUGH THE GREAT BASIN.
OUR FORECAST AREA WILL LARGELY BE BETWEEN THESE FEATURES BUT THEY
WILL CAUSE US TO KEEP A LINGERING SLIGHT CHANCE GOING...MAINLY OVER
SOUTH-CENTRAL ARIZONA. A MUCH WEAKER SHORT WAVE IS ADVERTISED TO
SLIDE DOWN FROM THE NORTH ON WEDNESDAY FOR SLIGHT CHANCES LIMITED TO
THE HIGHER TERRAIN OF SOUTH-CENTRAL AZ. SNOW LEVELS DROP TO NEAR
5000 FEET...INITIALLY OVER SOUTHEAST CALIFORNIA MONDAY MORNING THEN
EASTWARD TO SOUTH-CENTRAL ARIZONA BY EARLY MONDAY EVENING. THE
PRECIP WILL LARGELY BE OVER BY THE TIME THE SNOW LEVELS GET THAT LOW
AND THUS NO NOTABLE ACCUMULATIONS FOR OUR AREA...PERHAPS A DUSTING.
MADE A NUMBER OF TWEAKS TO THE POP/QPF GRIDS BUT NO MAJOR
ALTERATIONS FROM PREVIOUS PACKAGE. AS FOR TEMPERATURES...ANTICIPATE
LITTLE CHANGE OVER SOUTHEAST CALIFORNIA MONDAY SINCE THEY HAVE
ALREADY BEEN GETTING PRECIP. IT WILL BE MUCH COOLER THOUGH EAST OF
THE LOWER COLORADO RIVER VALLEY...ESPECIALLY SOUTH-CENTRAL ARIZONA.
IN FACT...PHOENIX AREA HIGH TEMP MONDAY MAY OCCUR BEFORE SUNRISE
WITH AFTERNOON PEAK IN THE 50S. WITH THE FOLLOW ON DISTURBANCES
TUESDAY AND WEDNESDAY...TEMPS WILL BE SLOW TO WARM UP THE FIRST HALF
OF THE WEEK.

FROM MID-WEEK ONWARD...RIDGING WILL TAKE HOLD ACROSS THE REGION AND
CONSEQUENTLY TEMPERATURES WILL ALSO CLIMB BACK ABOVE NORMAL BY LATE
WEEK INTO THE WEEKEND.

&&

.AVIATION...
SOUTH-CENTRAL ARIZONA INCLUDING KPHX...KIWA AND KSDL...
A MOIST COLD FRONT APPROACHING FOR MONDAY MORNING. CLOUDS AND
SHOWERS WILL SLOWLY INCREASE AFTER 08Z MON...REACHING A PEAK AROUND
18Z MON. AS A RESULT...BKN CLDS BETWEEN 6-8 THSD AGL THROUGH 15Z
MON...THEN LOWERING TO NEAR 2 THSD BKN IN MODERATE RAIN AT TIMES
BETWEEN 15Z AND 21Z MON. SOUTH WIND 6 TO 8 KNOTS...BECOMING GUSTY
NEAR 20 KNOTS AROUND 17Z WITH SHOWERS.


SOUTHEAST CALIFORNIA AND SOUTHWEST ARIZONA INCLUDING KIPL AND KBLH...
A MOIST COLD FRONT MOVING THROUGH THE AREA BETWEEN 07Z-12Z MON.  AS
A RESULT...OCCASIONAL MVFR CONDITIONS IN CLOUDS AND RAIN. CIGS NEAR
3 THSD AGL WITH OCCASIONAL MODERATE RAIN THROUGH 11Z...CLOUDS
LIFTING AND RAIN ENDING AFTER 13Z MON.

AVIATION DISCUSSION NOT UPDATED FOR AMENDED TAFS.

&&

.FIRE WEATHER...
TUESDAY THROUGH SATURDAY...
COOL AND STILL RELATIVELY MOIST CONDITIONS WILL BE PRESENT ON
TUESDAY WITH LINGERING RAIN CHANCES ACROSS SOUTH-CENTRAL ARIZONA.
WEAK HIGH PRESSURE AND DRYING CONDITIONS WILL MOVE IN STARTING
WEDNESDAY. TEMPERATURES WILL WARM BACK TOWARD NORMALS BY THURSDAY
AND THEN A FEW DEGREES ABOVE NORMAL FOR FRIDAY AND SATURDAY. WINDS
WILL BE RELATIVELY LIGHT THROUGH THE PERIOD...BUT AFTERNOON
BREEZINESS IS EXPECTED ON WEDNESDAY THROUGH FRIDAY. MINIMUM
HUMIDITIES WILL DROP INTO THE 20S ON WEDNESDAY AND THEN INTO THE
TEENS EACH AFTERNOON STARTING THURSDAY.

&&

.PSR WATCHES/WARNINGS/ADVISORIES...

AZ...FLOOD WATCH FROM 5 PM TODAY THROUGH 5 PM MONDAY AZZ020>021.
     FLOOD WATCH FROM MIDNIGHT TONIGHT THROUGH 8 PM MONDAY
     AZZ022>024.

CA...FLOOD WATCH UNTIL 12 PM MONDAY CAZ031>033.

&&

$$

VISIT US ON FACEBOOK...TWITTER...AND AT WEATHER.GOV/PHOENIX

DISCUSSION...VASQUEZ/AJ/LEINS
AVIATION...VASQUEZ
FIRE WEATHER...KUHLMAN









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