Small Craft Advisory 606, NWS Anchorage, AK

SOUTHCENTRAL AND SOUTHWEST ALASKA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE ANCHORAGE AK
144 PM AKDT MON AUG 11 2014

.ANALYSIS AND UPPER LEVELS...THE UPPER LOW OVER THE WESTERN GULF
IS BEGINNING TO OPEN UP INTO A TROUGH AND LIFT NORTHWARD TOWARD
SOUTHCENTRAL. THE RELATIVELY COLD AIR ALOFT (-20 DEGREES AT
500MB) UNDERNEATH THE LOW/TROUGH WILL INCREASE INSTABILITY OVER
SOUTHCENTRAL AS IT CONTINUES NORTHWARD THIS AFTERNOON AND EVENING.
MEANWHILE A LARGE VERTICALLY STACKED LOW IS TRACKING QUICKLY
EASTWARD ALONG THE ALEUTIANS WITH ITS ASSOCIATED FRONTAL SYSTEM
APPROACHING SOUTHWEST ALASKA AND KODIAK ISLAND. A STRONG
SUBTROPICAL JET ON THE BACK SIDE OF THE LOW IS CAUSING THE UPPER
TROUGH TO DIG SOUTHEASTWARD ACROSS THE NORTH PACIFIC WILL
EVENTUALLY SET UP A STRONG SOUTHERLY JET ACROSS THE GULF OF ALASKA
INTO SOUTHCENTRAL.

&&

.MODEL DISCUSSION...MODELS ALL SHOW A SIMILAR EVOLUTION OF THE
SYNOPTIC PATTERN OVER THE NEXT FEW DAYS. THE REAL FORECAST
CHALLENGE (AS HAS BEEN THE CASE IN RECENT DAYS) WILL BE TIMING AND
STRENGTH OF THE MANY SHORT-WAVES ROUNDING THE UPPER LOW AND TROUGH
AFFECTING THE SOUTHERN MAINLAND OF ALASKA. MODELS HAVE VERY LITTLE
SKILL IN HANDLING OF THESE FEATURES VERY FAR OUT IN TIME.
PLUS...IF ONE FEATURE ENDS UP BEING STRONGER THAN EXPECTED IT CAN
HAVE AN EFFECT ON EVERYTHING UPSTREAM. BASED ON JET DYNAMICS AND
EXTENT OF MOISTURE/THERMAL ADVECTION THERE IS HIGH CONFIDENCE IN
PERIODS OF HEAVY RAIN ALONG THE GULF COAST. FORECAST CONFIDENCE
OVERALL IN TIMING OF PRECIP EVENTS AND PROBABILITY OF RAIN INLAND
IS LOW. WILL HAVE TO WAIT UNTIL CAN BETTER IDENTIFY INDIVIDUAL
FEATURES EMBEDDED IN THE UPPER FLOW.

&&

.SHORT TERM FORECAST...

SOUTHCENTRAL ALASKA...THE ATMOSPHERE WILL REMAIN FAVORABLE FOR
CONVECTIVE ACTIVITY THROUGH EARLY TUESDAY MORNING AS THE COLD
UPPER TROUGH LIFTS NORTHWARD ACROSS SOUTHCENTRAL. THE BEST CHANCE
FOR THUNDERSTORMS WILL BE RIGHT OUT AHEAD OF THE TROUGH ACROSS THE
COPPER BASIN AND SUSITNA VALLEY...ESPECIALLY ALONG HIGHER TERRAIN.
THERE WILL BE LITTLE OR NO BREAK AT ALL BETWEEN THIS EXITING TROUGH
AND CYCLONIC FLOW OUT AHEAD OF THE BERING LOW. IN FACT MODELS ARE
TRENDING WETTER WITH APPROACHING FRONTAL SYSTEM AND KEYING IN ON
AN UPPER SHORT-WAVE HEADING FOR COOK INLET AND THE KENAI PENINSULA
TOMORROW AFTERNOON. PRESSURE GRADIENTS ALONG THE NORTH GULF COAST
WILL BEGIN TO TIGHTEN AS THE FRONTAL SYSTEM ENTERS THE GULF...BUT
IT DOESN'T LOOK QUITE STRONG ENOUGH TO PRODUCE COMPLETE DOWNSLOPE
DRYING. IN ADDITION...FLOW ALOFT WILL BE SOUTHERLY SO WOULD EXPECT
A GOOD CHANCE OF AT LEAST SOME HIGH BASED RAIN IN THE LEE OF THE
KENAI AND CHUGACH MOUNTAINS. DOWNSLOPING WILL INCREASE TUESDAY
NIGHT AND WEDNESDAY AS THE FRONTAL SYSTEM MOVES TO THE NORTH GULF
COAST. WITH STRONG SOUTHERLY FLOW ALOFT THE FRONT WILL PUSH
ONSHORE...WHICH MEANS STRONG PRE-FRONTAL WINDS WILL PUSH ALL THE
WAY INTO PRINCE WILLIAM SOUND. WITH A LONG FETCH OF TROPICAL
MOISTURE AND MODERATE TO STRONG DYNAMICAL FORCING EXPECT HEAVY
RAIN ALONG THE GULF COAST. WATER LEVELS ARE RUNNING HIGH IN THE
KENAI LAKE/KENAI RIVER SYSTEM...SO WILL LIKELY NEED TO ISSUE A
FLOOD ADVISORY AT SOME POINT IN TIME. A SERIES OF UPPER WAVES WILL
AFFECT THE REGION AS WE CONTINUE THROUGH THE WEEK...WHILE THE
FIREHOUSE OF TROPICAL MOISTURE REMAINS POINTED TOWARD SOUTHCENTRAL.
IN THIS SORT OF ENVIRONMENT INLAND (INCLUDING LEE SIDE OF THE
COASTAL RANGE) IS LIKELY TO SEE PERIODS OF LIGHT RAIN OR
SPRINKLES.

SOUTHWEST ALASKA...MORE STABLE AIR IS MAKING ITS WAY ONSHORE AS
THE OCCLUDED FRONT APPROACHES FROM THE BERING SEA. THUS...THE MAIN
THREAT FOR THUNDERSTORMS THIS AFTERNOON AND EVENING WILL BE ALONG
THE ALASKA RANGE AND INTO THE KUSKOKWIM VALLEY WHERE THERE IS SOME
CLEARING AT THE MOMENT AND SOME UPPER LEVEL FORCING FROM THE
UPPER LOW AND TROUGH LIFTING NORTHWARD OVER THE WESTERN GULF.
THE OCCLUSION WILL WEAKEN AS IT MOVES INLAND ACROSS SOUTHWEST
ALASKA TUESDAY...SO THIS DOES NOT LOOK LIKE A BIG RAIN PRODUCER.
A SERIES OF UPPER WAVES ROTATING OVER TOP OF THE LOW WILL CROSS
SOUTHWEST ALASKA THE NEXT FEW DAYS. FORECAST CONFIDENCE IN THE
TIMING AND STRENGTH OF THESE IS LOW...BUT GENERALLY THIS FLOW DOES
NOT PRODUCE BIG RAIN EVENTS.

BERING SEA/ALEUTIANS...THE UPPER LOW WILL SET UP SHOP SOUTH OF THE
ALASKA PENINSULA...SO THE BERING AND ALEUTIANS WILL DRY OUT OVER
THE NEXT COUPLE DAYS. UPPER LEVEL FLOW WILL REMAIN PROGRESSIVE
BUT MORE AMPLIFIED...WITH SYSTEMS TRACKING FARTHER SOUTH. THE NEXT
STORM WILL TRACK JUST SOUTH OF THE ALEUTIANS MID TO LATE IN THE
WEEK AND IMPACTS WON'T BE NEARLY AS WIDESPREAD AS THE LAST COUPLE
SYSTEMS.

&&

.LONG TERM FORECAST...THE OLD BERING/ALEUTIAN LOW STILL LOOKS TO BE
CENTERED NEAR BRISTOL BAY ON FRI BUT WILL BE FILLING SUBSTANTIALLY
AS THE SUPPORTING JET ALOFT MOVES AWAY. THE LOW WILL OPEN INTO A
TROUGH BY THE WEEKEND WITH SOME OF THE ENERGY LIFTING NORTHWARD
INTO THE CENTRAL AK INTERIOR AND SOME RESIDUAL ENERGY BECOMING
ENVELOPED INTO THE NEXT DOMINANT LOW APPROACHING FROM THE PACIFIC
SIDE OF THE ALEUTIAN CHAIN/AKPEN. MODEL RUN TO RUN CONSISTENCY HAS
IMPROVED TODAY AND MODEL AGREEMENT IS BETTER. THIS IS NOTABLE
CONSIDERING THE VARIOUS LOBES OF ENERGY THAT WILL BE INTERACTING
WITH EACH OTHER TO DEVELOP THIS AFOREMENTIONED NEW LOW IN THE
COMING DAYS. THE GFS/ECMWF/GEM TIMING OF SHORTWAVE ENERGY DROPPING
DOWN PAST THE WESTERN/CENTRAL ALEUTIANS EARLY FRI IS SIMILARLY
DEPICTED...AND THIS CLOSE CLUSTERED TIMING CONTINUES AS THIS
SHORTWAVE ENERGY INTERACTS WITH THE REMNANTS OF TYPHOON GENEVIEVE
GETTING CAUGHT UP IN THE MID TO UPPER-LVL FLOW.

FORECAST CHALLENGES WILL SURROUND THE TRACK OF THIS LOW. THE
OPERATIONAL GFS IS ON THE EASTERN EDGE OF THE SOLUTION ENVELOPE
TRACKING THE LOW TOWARDS SOUTHERN REACHES OF THE GULF BY EARLY
SUN. THE GEM IS FURTHEST WEST...NEAR THE AKPEN AT A COMMENSURATE
TIME. THE LONG TERM FORECAST HAS LEANED ON WPC AND ECMWF GUIDANCE
WITH SOME SUPPORT FROM GFS ENSEMBLES...WHICH NESTLES A MIDDLE
GROUND SOLUTION. THE JET STREAM FAVORS ACTIVE WEATHER FOR THE
COASTAL AREAS OF THE SOUTHERN MAINLAND AND GULF OF AK...WITH A BIT
OF A RESPITE FOR THE BERING SEA IN THE LONG TERM FORECAST PERIOD.

&&

.AFC WATCHES/WARNINGS/ADVISORIES...
PUBLIC...NONE.
MARINE...GALE WARNING 119.
FIRE WEATHER...NONE.

&&

SEB/JAM AUG 14