Area Forecast Discussion
Issued by NWS Anchorage, AK

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FXAK68 PAFC 030018
AFDAFC

SOUTHCENTRAL AND SOUTHWEST ALASKA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE ANCHORAGE AK
418 PM AKDT WED SEP 2 2015

.ANALYSIS AND UPPER LEVELS...
THE RIDGING THAT BROUGHT CLEAR SKIES AND COOL TEMPERATURES TO MUCH
OF THE SOUTH MAINLAND OVERNIGHT CONTINUES TO STRETCH FROM THE
SOUTHEAST MAINLAND INTO THE EASTERN GULF AND THE PANHANDLE. THIS
RIDGE IS BEING PUSHED EASTWARD BY A 100 KT JET STREAK OVER THE
NORTH PACIFIC THAT IS MOVING NORTHEAST TOWARDS THE SOUTHWEST
COAST. A SERIES OF SHORTWAVES ARE RIDING ALONG THIS JET
STREAK. THE FIRST WAS ASSOCIATED WITH A FRONTAL SYSTEM THAT MOVED
FROM THE CENTRAL BERING ONTO THE SOUTHWEST COAST OVERNIGHT...AND IS
BRINGING WIDESPREAD RAIN FROM BRISTOL BAY NORTHWARD INTO THE
KUSKOKWIM VALLEY. UPSTREAM OF THIS...A STRONGER SHORTWAVE PASSING
OVER THE EASTERN ALEUTIANS IS BRINGING ANOTHER ROUND OF RAIN TO
THAT AREA. THIS SHORTWAVE WILL CONTINUE TO PRESS TOWARDS THE
SOUTHWEST COAST THIS EVENING...AND HELP TO REINVIGORATE RAINFALL
THROUGHOUT THE SOUTH MAINLAND. THESE SHORTWAVES ARE ASSOCIATED
WITH A BROAD AND NEARLY STATIONARY UPPER LEVEL LOW CENTERED JUST
EAST OF THE KAMCHATKA PENINSULA. THIS LOW WILL SLOWLY DRIFT TO THE
EAST OVER THE NEXT FEW DAYS...AND HELP KEEP ACTIVE WEATHER WITH
CYCLONIC FLOW ALOFT FROM THE BERING SEA INTO THE SOUTH MAINLAND.

&&

.MODEL DISCUSSION...
THE MODELS REMAIN IN GOOD SYNOPTIC AGREEMENT WITH THE GENERAL PATTERN
THROUGH THE WORK WEEK...HOWEVER THE DEVIL REMAINS IN THE DETAILS
WITH REGARDS TO TIMING OF THE RAINFALL THROUGHOUT THE SOUTHWEST
MAINLAND. THERE IS SLIGHTLY BETTER AGREEMENT WITH THE ONSET OF
RAINFALL OVER SOUTHCENTRAL LATE THURSDAY MORNING...BUT THEY STILL
ARE STRUGGLING WITH HOW QUICKLY TO BRING THIS RAINFALL NORTH OF
ANCHORAGE. THE NAM/EC HAVE BOTH BEEN CONSISTENT IN A SLIGHTLY
FASTER SOLUTION THAN THE GFS/GEM...HOWEVER BY THURSDAY AFTERNOON
ALL THE MODELS HAVE GOOD CONSENSUS THAT WIDESPREAD RAIN SHOULD BE
FALLING THROUGHOUT MUCH OF THE SOUTH MAINLAND (WITH THE EXCEPTION
OF THE COPPER RIVER BASIN). FOR NOW THE FORECAST WILL CONTINUE TO
REFLECT THE SLIGHTLY FASTER SOLUTION. OTHERWISE THEY HAVE COME
INTO BETTER AGREEMENT SINCE YESTERDAY IN BOTH THE TIMING AND
INTENSITY OF A WEAK TRIPLE POINT LOW NEAR KODIAK ISLAND THURSDAY
NIGHT...ALLOWING FOR WINDS TO BE INCREASED OVER THE NORTH GULF
COAST TODAY. HOWEVER...WITH REGARDS TO RAINFALL THE MODELS
CONTINUE TO STRUGGLE WITH PLACING INDIVIDUAL WAVES FROM THE UPPER
LEVEL TROUGH THAT WILL SETUP ON FRIDAY...SO THE RAINFALL FORECAST
FOR FRIDAY REMAINS PAINTED WITH A BROAD BRUSH (A CHANCE OF RAIN
FOR SOUTHCENTRAL WITH LIKELY OR GREATER CHANCES ALONG THE NORTH
GULF COAST AND FURTHER EAST).

&&

.FIRE WEATHER...
FIRE WEATHER CONCERNS WILL CONTINUE TO DIMINISH THIS EVENING AS
CLOUDS INCREASE AND MOIST SOUTHWESTERLY FLOW DEVELOPS AHEAD OF THE
INCOMING RAIN. SKIES WILL REMAIN CLEAR THIS EVENING OVER THE
COPPER RIVER BASIN...BUT WITH LIMITED HEATING THIS LATE IN THE
SEASON AND WEAK SURFACE WINDS DRY CONDITIONS ARE NOT EXPECTED. BY
THURSDAY AFTERNOON AND EVENING...RAINFALL SHOULD SPREAD THROUGHOUT
SOUTHCENTRAL INCLUDING THE COPPER RIVER BASIN...ALLOWING FOR COOL
AND MOIST CONDITIONS TO SETUP THAT WILL PERSIST INTO FRIDAY. WEAK
RIDGING WILL THEN DEVELOP ON SATURDAY ALLOWING FOR PARTIAL
CLEARING SKIES...BUT WITH CONTINUED COOL TEMPERATURES AND LIGHT
WINDS FIRE WEATHER CONDITIONS SHOULD REMAIN LOW.

&&

.HYDROLOGY...
RISING WATER LEVELS AT THE SNOW LAKE (A GLACIER DAMMED LAKE ON
THE KENAI PENINSULA) TODAY IS INDICATING THAT THE LAKE IS
BEGINNING TO RELEASE INTO KENAI LAKE AND THE KENAI RIVER. AS A
RESULT A FLOOD WATCH HAS BEEN ISSUED FOR THE KENAI RIVER FROM
KENAI LAKE DOWNSTREAM TO THE RUSSIAN RIVER AS WATER LEVELS MAY
RISE TO MINOR FLOOD STAGE BETWEEN THURSDAY EVENING AND SATURDAY
MORNING. IN ADDITION...DUE TO CONCERNS ALONG LOW LYING AREAS OF
KENAI LAKE (INCLUDING THE PRIMROSE CAMPGROUND AREA) A FLOOD
ADVISORY HAS ALSO BEEN ISSUED. CONDITIONS THROUGHOUT THE AREA ARE
CURRENTLY EXPECTED TO BEGIN TO IMPROVE THIS WEEKEND AS THE RELEASE
AT KENAI LAKE ENDS.

&&

.SHORT TERM FORECAST SOUTHCENTRAL ALASKA (DAYS 1 AND 2)...
THE EXPECTED PATTERN SHIFT FROM THE LAST FEW DAYS OVER
SOUTHCENTRAL ALASKA IS UNDERWAY WITH INCREASING CLOUDS OVER THE
REGION THIS AFTERNOON. THE FLOW ALOFT IS BEGINNING TO TURN
SOUTHWEST OVER THE AREA AS SEEN ON THE AFTERNOON SOUNDINGS AND
SATELLITE DATA...BRINGING A MOIST AIRMASS INTO THE REGION WITH
CURRENT SATELLITE DATA INDICATING PRECIPITABLE WATER VALUES OF
AROUND 1.5-1.85 INCHES /ABOUT 150-200 PERCENT OF NORMAL/. RAINFALL
HAS ALREADY STARTED ON KODIAK ISLAND AND THE LEADING EDGE OF THE
RAIN SHIELD IS VISIBLE ON RADAR OVER THE BARREN ISLANDS THIS
AFTERNOON. RAINFALL SHOULD CONTINUE TO PROGRESS EASTWARD OVERNIGHT
WITH RAIN STARTING IN THE ANCHORAGE AREA AFTER MIDNIGHT AND
PROGRESSING THURSDAY AFTERNOON INTO THE NORTHERN GULF COAST AND
THE COPPER RIVER BASIN. CONFIDENCE IN THE EXACT LOCATION AND
AMOUNTS OF THE HEAVIEST RAINFALL TOTALS REMAIN BELOW AVERAGE...BUT
OVERALL 48 HOUR RAINFALL TOTALS OF UP 0.50-0.75 INCHES ARE
EXPECTED...MAINLY OVER COASTAL AREAS.

AS A WEAK AND SHALLOW LOW DEVELOPS IN THE NORTHERN GULF
THURSDAY AFTERNOON...THE FOCUS OF THE STEADY RAINFALL WILL SHIFT
TO PRINCE WILLIAM SOUND AND INTO THE COPPER RIVER BASIN WITH
SHOWERS EXPECTED IN THE WESTERN PART OF THE AREA.

&&

.SHORT TERM FORECAST SOUTHWEST ALASKA (DAYS 1 AND 2)...
RAIN WILL CONTINUE OVER MUCH OF THE SOUTHWEST MAINLAND TONIGHT...AS
THE REMNANTS OF A FRONTAL SYSTEM OVER THE MAINLAND CONTINUE TO
WEAKEN THROUGH THE EVENING. THE ONLY EXCEPTION TO THIS BEING THE
KUSKOKWIM DELTA...WHERE THE FRONTAL PRECIPITATION TAPERED OFF
EARLIER TODAY AND SHIFTED NORTHWARD INTO THE NORTHWEST INTERIOR.
MULTIPLE WAVES TRACKING UP FROM THE NORTH PACIFIC WITH A SURFACE
CIRCULATION CROSSING THE ALASKA PENINSULA TONIGHT WILL REINFORCE
RAINS OVER THE MUCH OF THE SOUTHWEST MAINLAND TONIGHT...INCLUDING
THE KUSKOKWIM DELTA BY TOMORROW MORNING AS THE LOW TRACKS FURTHER
INTO BRISTOL BAY. RAIN WILL THEN TRANSITION TO MORE SHOWERY
CONDITIONS OVER THE AREA...AS A TROUGH OVER THE BERING EVENTUALLY
PUSHES THE BULK OF PRECIPITATION FURTHER EASTWARD AND MAKES WAY
FOR ANOTHER SET OF UPPER LEVEL WAVES TO TRACK INTO THE SOUTHWEST
MAINLAND FROM THE EASTERN BERING BY FRIDAY MORNING. THESE WAVES
WILL KEEP SCATTERED TO NUMEROUS SHOWERS OVER THE MAINLAND INTO THE
WEEKEND.

&&

.SHORT TERM FORECAST BERING SEA/ALEUTIANS (DAYS 1 AND 2)...
THE LARGE UPPER LEVEL LOW SLOWLY ENTERING THE WESTERN BERING WILL
CONTINUE ITS EASTWARD PROGRESSION INTO THE CENTRAL BERING BY THE
WEEKEND. TROUGHS ASSOCIATED WITH THIS LARGE UPPER LOW WILL ROTATE
AROUND THE BASE OF THE LOW...KEEPING SHOWERY CONDITIONS ALONG THE
WESTERN AND CENTRAL ALEUTIANS/BERING THROUGH THE END OF THE WEEK.

&&

.LONG TERM FORECAST (DAYS 3 THROUGH 7)...

RAIN WILL CONTINUE TO TAPER OFF ACROSS SOUTHERN ALASKA FRIDAY
EVENING THROUGH THE START OF THE UPCOMING WEEKEND AS A POTENT
SHORTWAVE AND A WEAK SURFACE REFLECTION SHIFT INTO THE EASTERN
GULF OF ALASKA BEFORE FALLING APART AT THE PANHANDLE. SHORTWAVE
RIDGING WILL DEVELOP BRIEFLY BEHIND THIS PARTING SHORTWAVE AS A
SERIES OF UPPER LEVEL LOWS IN THE BERING SEA DROP SOUTH AND CREATE
A ZONAL UPPER LEVEL PATTERN VS THE AMPLIFIED PATTERN SEEN AS OF
LATE. THIS ZONAL FLOW WILL KEEP MUCH OF ALASKA IN A FAVORABLE
LOCATION FOR STORMS TO TRACK FROM THE BERING SEA INTO INTERIOR
LOCATIONS OF ALASKA. THE MOST NOTABLE CHANGE FROM YESTERDAY IS
THAT ALL GUIDANCE IS NOW HINTING AT THE ZONAL FLOW AMPLIFYING
TOWARD THE MIDDLE OF NEXT WEEK WHICH WILL ALLOW THE PHASING OF THE
AMPLIFYING TROUGH OVER THE NORTH PACIFIC TO MERGE WITH THE
NORTHERN STREAM. THE PHASING OF THESE SYSTEMS LOOKS TO USHER IN
ANOTHER UNSEASONABLY COLD AIR MASS FROM THE ARCTIC AS AN ARCTIC
TROUGH DIVES SOUTH. THERE IS STILL A LOT OF UNCERTAINTY WITH THE
TIMING OF THIS HAPPENING BUT GUIDANCE HAS BEEN HINTING AT THIS FOR
MULTIPLE DAYS AND MODELS ARE GETTING CLOSER TO A SOLUTION EACH
DAY. IT LOOKS AS IF NEAR NORMAL TEMPERATURES WILL PERSIST THROUGH
THE BEGINNING OF NEXT WEEK AND WOULD DROP A GOOD BIT BELOW NORMAL
BY THE MIDDLE OF NEXT WEEK SHOULD THIS ARCTIC TROUGH DIVE SOUTH
OVER SOUTHWEST AND SOUTHCENTRAL ALASKA.

&&


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

&&

$$

SYNOPSIS AND MODEL DISCUSSION...DEK
SOUTHCENTRAL ALASKA...RF
SOUTHWEST ALASKA/BERING SEA/ALEUTIANS...TP
LONG TERM...MC



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