Area Forecast Discussion
Issued by NWS Anchorage, AK

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FXAK68 PAFC 171401
AFDAFC

Southcentral and Southwest Alaska Forecast Discussion
National Weather Service Anchorage AK
501 AM AKST Sat Nov 17 2018

.ANALYSIS AND UPPER LEVELS...
Upper levels show ridging continuing to push east over the Alcan
border, while a low sits in the northern Bering. Showery
conditions, mostly rain, are still occurring out west along the
Chain and Southwest as a surface low makes its way eastward.

For Southcentral, widespread precipitation was observed overnight
and continues along the north Gulf coast associated with the
front moving across the Gulf. Radar imagery shows precipitation
pushing over the coastal areas, the Kenai Peninsula, and into the
Anchorage Bowl. Rainfall was the main p-type observed from Kodiak
to Cordova, with the exception being Valdez and Gulkana, which
continues to see snow. The Copper River Basin will be the main
concern this morning with the Winter Weather Advisory issued for
mixed precipitation with snow changing over to freezing rain this
morning.



&&

.MODEL DISCUSSION...
Based on satellite imagery and METAR data, the models initialized
fairly well. The one caveat being the precipitation over the
Kenai Peninsula which the NAM and GFS did not have. There is
decent consensus with the synoptic pattern for 36 hrs. However,
there are some pesky timing issues with the precipitation shield
and the p-type. Past 36 hrs, the the models diverge significantly
at both the surface and 500 mb. The divergence in the guidance is
not limited to the Bering, its also very glaring over the Gulf of
Alaska.

&&

.AVIATION...
PANC...VFR conditions and light winds will persist.

&&

.SHORT TERM FORECAST SOUTHCENTRAL ALASKA (Days 1 and 2/Today
through Sunday night)...
A strong negatively tilted upper level trough over the northern
Gulf will lift northward across eastern Prince William Sound this
morning and the Copper River Basin this afternoon, with rain along
the coast and mostly snow inland. A Winter Weather Advisory for
snow and freezing rain remains in effect until Noon for the
Copper River Basin from Glennallen southward. However, the extent
of warming aloft does not look as significant as previous model
runs indicated. Thus, have backed off freezing rain this morning,
confining it to the southeastern basin, from about Chitina to
McCarthy. Areas north of Glennallen will see snow as well, but
expect very little accumulation. Warming aloft will continue
northward this afternoon through tonight, but the heart of the
basin will dry out.

Meanwhile, a weaker short-wave trough back over Cook Inlet and the
Kenai Peninsula will lift northward early this morning, producing
a brief period of light precipitation. While temperatures on the
Kenai are all above freezing, much of Anchorage and the Mat-Su
remain below freezing. Thus, could see some light freezing rain
or sprinkles. Based on short duration and light intensity do not
expect any significant icing or impacts from this.

As we head through the next couple days, a series of short-wave
troughs will traverse Southcentral as a high amplitude long-wave
trough moves in from the Bering Sea. Forecast confidence remains
low as models continue to struggle with the details of each
feature moving through the upper level trough. There are also
major differences on the track and strength of a surface low
moving up from the northeast Pacific Sunday through Sunday night.
So what can we say with confidence about the forecast? Coastal
areas will remain wet no matter which solution pans out. Surface
temperatures will be on a steady rise across the region today
through tonight. This means that wherever precipitation does
fall, it will be primarily in the form of rain. The only areas
which look to remain cold enough for all snow are the far northern
Susitna Valley and Copper River Basin (near the Alaska Range).

&&

.SHORT TERM FORECAST SOUTHWEST ALASKA (Days 1 and 2)...
(Today through Sunday)

The weather will remain active through tomorrow, with wintry type
precipitation and amounts being the main forecast challenge. A
trio of low pressure systems to our west and southwest continues
to funnel warm moist air into the region this morning, and these
lows will consolidate into a single low moving northward through
the AKPEN into Bristol Bay by this evening. This system will swing
a warm front northward reaching the Nushagak Hills shortly after
sunset. This will let some of the cold air at the surface remain
entrenched in the valleys to the north of these hills and east of
the Kilbuck Mountains through Sunday morning. Additional vort
maxes crossing the area aloft will provide bursts of ascent which
will periodically aid in precipitation efficiency as well, with
some of the latest numerical data suggesting a few inches of snow
along with up to two tenths of an inch of ice are likely in the
valleys for this area.

Further south and west, precipitation will be in the form of rain,
except for some locations near/north of the Kuskokwim river across
the delta, where temperatures will be marginally sufficient for a
wintry mix or periods of freezing rain to persist through the
evening. However, temporal and area coverage of this threat is low
enough to preclude any headline issuance. Look for periods of
precipitation to continue through Sunday in this active weather
pattern.

&&

.SHORT TERM FORECAST BERING SEA/ALEUTIANS (Days 1 and 2)...
(Today through Sunday)

Several vort maxes in the cyclonic flow aloft will continue to
produce showery type precipitation across the region through
Sunday. Winds will mainly be of the small craft variety, with a
few periods of localized sustained gales possible. This will be a
result of increased dry and cold advection on the backside of the
disturbances, which increases the downward mixing potential,
leading to the increase in winds. Wave heights will also increase
across central portions of the Bering as this region will have a
long fetch length and be closer to the path of these
disturbances.

&&

.LONG TERM FORECAST (Days 3 through 7/Monday Night through
Friday)...

Disorganization and uncertainty continues for the long term.
Solutions are at least similar to yesterday in that we start next
week with troughing over southcentral AK and a closed upper level
low near SW Alaska. However, models are struggling with their
placement of the low over SW Alaska with variation in the
troughing extending from this low over the eastern and central
Aleutians. This means any frontal systems associated with this
low will be placed much differently at the surface. Therefore, we
still know weather will be active for much of the forecast area,
but the details are still very fuzzy.

Going forward through next week, a broad area of troughing
remains over southcentral AK, with disorganized lows spinning
through this trough. Once again, too far in out in time to predict
particulars of these systems. Ridging exists over the central
Aleutians, while a trough is over the western Aleutians. This
trough/ridge pattern will be slowly working its way east through
the week, with models hinting at the ridge arriving over
southcentral AK on Friday.

&&


.AFC WATCHES/WARNINGS/ADVISORIES...
PUBLIC...Winter Weather Advisory 141 152.
MARINE...Gale Warning 119 120 130 131 414.

&&

$$

SYNOPSIS AND MODEL DISCUSSION...PAMBER
SOUTHCENTRAL ALASKA...SEB
SOUTHWEST ALASKA/BERING SEA/ALEUTIANS...PD
LONG TERM...BB



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