Flood in Alaska, July 25 to 26, 2015

SOUTHCENTRAL AND SOUTHWEST ALASKA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE ANCHORAGE AK
250 PM AKDT SAT JUL 25 2015

.ANALYSIS AND UPPER LEVELS...
UPPER LEVEL FLOW IS PROGRESSIVE FROM SIBERIA ACROSS THE BERING SEA
AND ALASKA WITH STRONG RIDGING TO THE SOUTH ACROSS MUCH OF THE
NORTH PACIFIC. THE MAIN FEATURE OF INTEREST IS A WEAK VERTICALLY
STACKED LOW NEAR NUNIVAK ISLAND WITH AN UPPER TROUGH TRAILING
SOUTHWESTWARD ACROSS THE CENTRAL BERING SEA AND ALEUTIANS. A
100KT+ SUBTROPICAL JET ORIENTED WSW TO ENE ACROSS THE SOUTHEASTERN
BERING SEA IS DRIVING THE LOW AND TROUGH EASTWARD TOWARD SOUTHCENTRAL
AND THE GULF. THE BULK OF RAIN WITH THIS STORM SYSTEM IS FOCUSED
OUT AHEAD OF THE UPPER LEVEL TROUGH AND ALONG A LOW LEVEL WARM
FRONT EXTENDING FROM THE EASTERN ALEUTIANS TO THE ALASKA PENINSULA.
PRECIP OUT AHEAD OF THIS IS MORE SHOWERY WITH WEAK INSTABILITY IN
PLACE OVER MUCH OF INTERIOR SOUTHWEST AND SOUTHCENTRAL ALASKA.
NORTHERLY FLOW IS BEGINNING TO STRENGTHEN ON THE BACK SIDE OF THE
UPPER TROUGH (AS A SHARP SHORT-WAVE RIDGE BUILDS INTO THE WESTERN
BERING) LEADING TO SOME AMPLIFICATION OF THE UPPER TROUGH. THIS
PROCESS ALONG WITH THE OVERALL DYNAMIC NATURE OF THE UPPER LEVEL
FEATURES WILL ULTIMATELY LEAD TO DEVELOPMENT OF MORE WIDESPREAD
SHOWERS OR RAIN ACROSS THE SOUTHERN MAINLAND THIS AFTERNOON AND
EVENING. THUS...AFTER STARTING THE MORNING WITH SUNNY AND WARM
CONDITIONS ACROSS MOST OF SOUTHCENTRAL...CONDITIONS WILL BE
DETERIORATING WITH TIME.

&&

.MODEL DISCUSSION...
MODELS REMAIN IN GOOD AGREEMENT WITH LARGE SCALE FEATURES. GENERALLY
PREFER THE HIGHER RESOLUTION GUIDANCE FOR BETTER HANDLING/REPRESENTATION
OF THE LOCAL SCALE WINDS AND QPF. FORECAST CONFIDENCE IS RELATIVELY
HIGH TONIGHT THROUGH SUNDAY NIGHT AS THE CURRENT STORM SYSTEM
CROSSES THE SOUTHERN MAINLAND AND GULF OF ALASKA. FORECAST CONFIDENCE
THEN DROPS BEHIND THIS SYSTEM (FOR MONDAY AND BEYOND) AS A BROAD
UPPER TROUGH SITS OVER THE SOUTHERN MAINLAND. THERE DOESN'T APPEAR
TO BE ANY PARTICULAR FEATURE TO GRASP ON TO. THIS WILL MAKE IT
DIFFICULT TO PINPOINT SHOWERS...CLOUD COVER...AND AS A RESULT OF
BOTH OF THESE FACTORS...TEMPERATURES.

&&

.FIRE WEATHER...
GOOD UPPER LEVEL DYNAMICS ALONG WITH GENERALLY WEAK LOW LEVEL FLOW
(TO MINIMIZE DOWNSLOPE DRYING) WILL PRODUCE WIDESPREAD WETTING
RAINS TONIGHT THROUGH SUNDAY FOR MUCH OF INTERIOR SOUTHWEST ALASKA
AND NEARLY ALL OF SOUTHCENTRAL ALASKA. IN THE WAKE OF THIS A
PERSISTENT UPPER TROUGH WILL REMAIN IN PLACE OVER THE REGION
THROUGH THE MIDDLE OF NEXT WEEK. THIS GENERALLY SUPPORTS A COOLER
AND CLOUDIER PATTERN WITH DIURNALLY DRIVEN SHOWERS AND A CHANCE
OF SOME WET THUNDERSTORMS. HOWEVER...WITH GENERALLY WEAK FEATURES
EMBEDDED WITHIN THE TROUGH THERE LIKELY WILL BE SOME SHORT PERIODS
OF SUNSHINE WITH WARMER/DRIER CONDITIONS ON A MORE LOCAL SCALE. IN
ADDITION...WITH A BUILDING RIDGE UPSTREAM OVER THE WESTERN BERING
FLOW OVER WESTERN ALASKA WILL BECOME MORE NORTHERLY. THIS WILL
TRANSPORT COOL AND DRIER AIR SOUTHWARD TO THE KUSKOKWIM DELTA AND
WESTERN PORTIONS OF BRISTOL BAY SUNDAY NIGHT INTO MONDAY.

&&

.SHORT TERM FORECAST SOUTHCENTRAL ALASKA (DAYS 1 AND 2)...
AN APPROACHING JET STREAK AND UPPER LEVEL TROUGH WILL QUICKLY
SHIFT THE SENSIBLE WEATHER REGIME THIS EVENING. A DISTURBANCE OUT
AHEAD OF THE MAIN TROUGH WILL TAP LINGERING INSTABILITY ALONG THE
MOUNTAINS...ESPECIALLY ALONG THE TALKEETNA MOUNTAINS AND ALASKA
RANGE. AS THE MAIN TROUGH MOVES CLOSER TONIGHT PRECIPITATION WILL
SHIFT FROM SHOWERS TO STEADY RAIN ACROSS MOST OF SOUTHCENTRAL.
WIDESPREAD RAIN WILL CONTINUE THROUGH THE OVERNIGHT HOURS INTO
SUNDAY AFTERNOON BEFORE PRECIPITATION CHANGES CHARACTER BACK TO A
SHOWERY OR PERIODS OF RAIN NATURE. THE UPPER LEVEL LOW HANGS
AROUND THROUGH MONDAY WHICH WILL KEEP MOST OF THE AREA UNDER A
MOIST AND CLOUDY REGIME. DETAILS ARE STILL MUDDLED AT THIS POINT
SINCE ANY EMBEDDED DISTURBANCES CAN NOT BE RESOLVED BY MODELS BUT
SHOWERS REMAIN POSSIBLE ACROSS MUCH OF THE AREA AS LONG AS
CYCLONIC FLOW REMAINS OVERHEAD. TEMPERATURES WILL REMAIN RIGHT
AROUND AVERAGE WITH GENERALLY LIGHT WINDS FOR THE NEXT COUPLE DAYS
EXCEPT FOR SOME GUSTY WINDS ALONG TURNAGAIN ARM AND HIGHER
ELEVATIONS.

&&

.SHORT TERM FORECAST SOUTHWEST ALASKA (DAYS 1 AND 2)...
RAIN WILL CONTINUE TO SPREAD ACROSS SOUTHWEST ALASKA TONIGHT AS
THE UPPER LEVEL LOW AT THE MOUTH OF THE KUSKOKWIM VALLEY ROTATES
INLAND THEN SLOWLY CROSSES SOUTHWEST ALASKA. THE UPPER LOW WILL
SHIFT EAST INTO SOUTH CENTRAL ALASKA SUNDAY AFTERNOON AND EVENING
WITH RAIN CHANGING OVER TO SHOWERS BEHIND IT. NORTHWESTERLY FLOW
ALOFT WILL PERSIST OVER THE AREA THROUGH MONDAY ALLOWING FOR
SHOWERS TO TAPER OFF TO A MORE SCATTERED COVERAGE OVER ALL BUT THE
EASTERN MOUNTAINS WHERE THEY WILL REMAIN NUMEROUS. A SMALL
SHORTWAVE TROUGH RIPPLING THROUGH THE NORTHWESTERLY FLOW MONDAY
AFTERNOON AND EVENING IN COMBINATION WITH MIDDAY SUN BREAKS WILL
BRING THE POTENTIAL FOR ISOLATED THUNDERSTORMS TO THE MIDDLE
KUSKOKWIM VALLEY AND PORTIONS OF THE GREATER BRISTOL BAY AREA.

&&

.SHORT TERM FORECAST BERING SEA/ALEUTIANS (DAYS 1 AND 2)...
A FRONTAL SYSTEM IS CURRENTLY STRETCHED FROM THE CENTRAL ALEUTIANS
EAST ACROSS THE SOUTHERN BERING AND ALASKA PENINSULA. THIS SYSTEM
WILL SHIFT SOUTH AND EAST TONIGHT AND TOMORROW AS THE SUPPORTING JET
STREAM WEAKENS AND SWINGS EAST AS WELL. A WEAK SHORTWAVE TROUGH
MOVING IN FROM THE NORTHWEST WILL CROSS THE PRIBILOFS SUNDAY NIGHT
AND THE EASTERN ALEUTIANS AND WESTERN TIP OF THE ALASKA PENINSULA
MONDAY. HIGH PRESSURE WILL PERSIST OVER THE WESTERN ALEUTIANS.

&&

.LONG TERM FORECAST (DAYS 3 THROUGH 7)...
SHOWERS DURING THE BEGINNING OF THE WEEK WILL PERSIST THROUGH THE
LATER HALF OF THE WEEK...AS MUCH OF SOUTHERN MAINLAND WILL REMAIN
UNDER A BROAD TROUGH WITH A SERIES OF SHORTWAVES TRACKING THROUGH
THE AREA. MODELS CONTINUE TO STRUGGLE WITH THE TRACKS/TIMING OF
THESE SHORTWAVES...WHICH HAS LEAD TO LOWER FORECAST CONFIDENCE ON
WHERE AND WHEN SHOWERS WILL DEVELOP.

FURTHER WEST...THE BERING AND ALEUTIANS WILL STAY IN A RELATIVELY
QUIETER PATTERN THROUGH TUESDAY. AN UPPER LEVEL RIDGE BUILDS
ACROSS THE WESTERN BERING WITH SCATTERED SHOWERS STILL POSSIBLE
ACROSS EASTERN AREAS DUE TO WEAK SHORTWAVES MOVING ALONG THE
BACKSIDE OF THE MAIN LOW MOVING INTO THE GULF. BY TUESDAY EVENING
THE MODELS BEGIN TO STRUGGLE WITH THE TIMING/TRACK AND COMPLEXITY
OF A NORTH PACIFIC LOW AND ASSOCIATED FRONT APPROACHING THE
WESTERN ALEUTIANS. THE SLIGHTLY FASTER AMERICAN MODELS BRING IN
SOME SCATTERED SHOWERS AND MILDLY GUSTY WINDS TO THE PACIFIC SIDE
OF THE WESTERN/CENTRAL ALEUTIAN CHAIN TROUGH THURSDAY...WHERE THE
ECMWF KEEPS THE SHOWERS AND WINDS FURTHER SOUTH. AS THE SYSTEM
BECOMES SPLIT BY THE HIGH OVER THE BERING...THE EASTERN
CIRCULATION BEGINS TO DEEPEN AND TRACK INTO THE GULF. HOWEVER...
MODEL TIMING ON WHEN THE ASSOCIATED FRONTAL RAINFALL AND WINDS IS
MORE THAN 12 HOURS OFF BETWEEN THE GFS AND EC. AS A RESULT...
FORECAST CONFIDENCE REMAINS LOW REGARDING THIS SYSTEM THROUGH THE
WEEKEND.

&&

.AFC WATCHES/WARNINGS/ADVISORIES...
PUBLIC...NONE.
MARINE...NONE.
FIRE WEATHER...NONE.

&&

SYNOPSIS AND MODEL DISCUSSION...SB
SOUTHCENTRAL ALASKA...ML
SOUTHWEST ALASKA/BERING SEA/ALEUTIANS...JR
LONG TERM...TP

SPECIAL WEATHER STATEMENT
NATIONAL WEATHER SERVICE ANCHORAGE AK
609 PM AKDT SAT JUL 25 2015

AKZ101-260415-
ANCHORAGE-
INCLUDING THE CITIES OF...ANCHORAGE...EAGLE RIVER...INDIAN...
EKLUTNA
609 PM AKDT SAT JUL 25 2015

...HEAVY RAIN SHOWERS OVER ANCHORAGE THIS EVENING...

OCCASIONAL HEAVY RAIN SHOWERS WILL CONTINUE OVER ANCHORAGE DURING
THE EARLY EVENING. CONSIDERABLE PONDING OF WATER ON ROADWAYS IS
EXPECTED IN SOME OF THESE SHOWERS...AS WELL AS REDUCED
VISIBILITIES. THESE SHOWERS WILL DIMINISH IN INTENSITY LATER THIS
EVENING...CHANGING TO STEADY BUT LIGHTER RAINFALL.

THOSE TRAVELING SHOULD BE PREPARED FOR DIFFICULT DRIVING
CONDITIONS IN HEAVIER SHOWERS.