Alaska Extended Forecast Discussion
NWS Weather Prediction Center College Park MD
650 PM EDT Tue Jul 06 2021
Valid 12Z Sat Jul 10 2021 - 12Z Wed Jul 14 2021
...Heavy rain threat for coastal Southcentral to the northern
Panhandle this weekend...
...Overview...
An upper low will slowly migrate from the New Siberian Islands
toward the North Pole during the period as ridging builds through
China to the Korean Peninsula. This results in split flow over the
high latitudes and a narrow path for a slowly-moving omega block
to decay over the state. Lead upper low out of the Arctic will be
the main focus for the period as it swings through the Gulf and
toward the Panhandle during the five-day period. Its attendant
cold front will move into the Panhandle late Sun into early Mon as
enough moisture gets drawn into coastal areas from the southwest.
Farther north, southern side of the westerly flow between the
North Slope and the Arctic will support another cold front moving
eastward and slowly southward. This will bring some rainfall to
northern areas mid-period.
...Guidance/Predictability Evaluation...
The 12Z GFS and ECMWF offered reasonable clustering with their
ensemble means and each other to use as a basis for the surface
progs (including the 12Z UKMET for days 4-6). The 12Z ECMWF was a
bit different (but not implausible) over the Gulf by Sunday as it
ejected a piece of its upper energy eastward into the northern
Panhandle leaving a weaker upper trough over the Gulf compared to
the GFS and ensembles. Otherwise, trended toward the ensemble
means for the latter days of the forecast given uncertainty in the
height pattern along 70N (perhaps in between the GFS and ECMWF)
but otherwise good clustering even to next Wed/D8.
...Weather/Threats Highlights...
Rainfall with the main Gulf system will move from west to east
along the coast on the north side of the surface low as moisture
gets wrapped back into/around the system. 1-2" of rainfall per day
will accompany the system, with enough progression to keep 1-2
days of steadier rainfall over any given area. Some higher local
amounts of 3-5" may be possible in favored southeastern-facing
areas of the Kenai peninsula and around Yakutat. Farther north,
available moisture over the North Slope will be well above normal
ahead of the cold front through the Beaufort Sea, promoting some
organized rainfall across the region. Models differed on amounts
with the GFS driest and ECMWF/UKMET wetter (preference for the
wetter side of the guidance spread).
Temperatures are forecast to be near to below normal over
southern/coastal areas into the Panhandle with the abundant cloud
cover and rainfall. In addition, areas around the Brooks Range may
see near to below normal temperatures due to increased cloud cover
and precipitation as well. Areas in between through the interior
will see near to above normal temperatures with more sunshine,
allowing temperatures to climb into the 70s and low 80s.
Fracasso
Hazards:
- Heavy rain across portions of southern mainland Alaska, Fri, Jul
9.
- Heavy rain across portions of southern mainland Alaska and the
Alaska Panhandle, Sat-Mon, Jul 10-Jul 12.
Additional 3-7 Day Hazard Information can be found on the WPC
medium range hazards chart at:
https://www.wpc.ncep.noaa.gov/threats/threats.php
WPC medium range Alaskan products including 500mb, surface
fronts/pressures progs and sensible weather grids can also be
found at:
https://www.wpc.ncep.noaa.gov/alaska/ak_5dayfcst500_wbg.gif
https://www.wpc.ncep.noaa.gov/alaska/akmedr.shtml
https://www.wpc.ncep.noaa.gov/alaska/ak_5km_gridsbody.html