616 
FXUS66 KPDT 011145
AFDPDT

Area Forecast Discussion
National Weather Service Pendleton OR
345 AM PST Thu Feb 1 2024

Updated Aviation Discussion

.SHORT TERM...Today through Saturday...Deep upper low pressure
continues to be the main focal point for weather over the next
several days as it slowly moves towards coastal Washington today
and then slides southward along the coastline towards California
through the weekend, while part of the troughing associated with
this low will split off and move eastwards into the Four Corners
region, becoming its own cutoff system by Saturday as well.

Under this pattern, a moist upper level flow of southerly is
expected today that will then become more westerly to
southeasterly across the CWA as the splitting troughing bisects
our forecast area. What will be noteworthy is that under this
pattern, QPF will begin to concentrate across the eastern side of
the CWA, particularly in the Wallowas. Widespread QPF totals for
the Blues and Ochoco-John Day Highlands of around a quarter to
half of an inch through Saturday afternoon are expected, while the
Wallowas can expect three-quarters to 1.5 inches. Much of the
heavier totals in the high terrain will translate to snow, with
moderate to high confidence (60-80%) in accumulations of 10-20
inches for the crests and peaks. By Saturday when snow levels can
descend low enough to reach the valleys, the bulk of the heaviest
QPF will be gone and should keep these areas from seeing 
significant snowfall. With that being said, hydrology could be a 
concern as snow melt off and periods of moderate to heavy rainfall
occur. The NBM indicates a 50-75% chance of QPF of or in excess 
of three-quarters of an inch, with some of this falling in the 
mid-elevations where it will remain rainfall until Friday into 
Saturday when snow levels drop below 4000 feet. With how long of 
a duration this event is over the next few days, don't see enough 
of a signal to warrant any sort of flood watch products 
currently, but local impacts can't be ruled out over the coming 
days.

Outside of precipitation, today should be the last day of
significantly warmer than normal temperatures as the troughing
brings a cooler airmass. There is high confidence (80%) that we 
will see a steady cooling trend, with highs by Saturday for our 
population centers in the mid 40's, around 7 to 10 degrees lower 
than today. Overnight lows going into Sunday morning will descend
into the low 20's to low 30's, bringing freezing conditions once 
again. Otherwise that ends sensible weather concerns as the 
splitting troughing loosens the overall pressure gradient, calming
winds and promoting much more benign conditions, though we all 
know that during the winter that could bring back the fog. 
Forecast soundings do show surface inversions possible once 
again, so that cannot be ruled out. Goatley/87


.LONG TERM...Sunday through Thursday...Forecast will show a slight 
chance to a chance of precipitation each day with a slight cooling 
trend during the long term period. Basically, the precipitation type 
will be rain or rain/snow mix in the lower elevations and snow in 
the mountains. Models are in general agreement for Sunday through 
Monday but differences are more noticeable by mid week.

On Sunday, a broad trough over the eastern Pacific and the PacNW is 
advertised by the models.  It is an unorganized trough, but the 
cyclonic flow aloft will bring a 15-30% chance of snow over the 
mountains.  The chance of precipitation will increase Sunday night 
to around 30-50% over eastern Oregon and 20-30% over far southern 
Washington due to an increasing southerly flow aloft and an embedded 
shortwave over Oregon. Snow levels will be around 2500-3000 feet, 
and there will be a brisk north-northeast wind for a bitter wind 
chill.  The system will continue to track northward across eastern 
Washington on Monday for a widespread 30-60% PoPs and a good chance 
for 1-3 inches of snow above 2500 feet.

As stated previously, model differences begin to emerge starting 
Tuesday. All show some form of a split flow off the coast as an 
upper low dives south into California. However, about 50% of the 
ensembles maintain an open trough over the west coast while the 
other models are either farther south with the low or pinch off the 
low with a westerly flow over the PacNW.  Snow levels will remain 
around 2500-3000 feet with 20-40% PoPs in the lower elevations and 
around 60% PoPs in the mountains.  

Confidence in the models and the extended forecast fades starting 
Wednesday. It's a matter if the California low tracks to the east, 
as shown by over 50% of the GEFS but less than 15% of the EC ENS and 
25% of the GEPS or has little movement with a continued split flow 
over the PacNW.  The deterministic ECMWF pushes the low eastward and 
builds a transitory ridge over WA/OR.  These differences play a 
role, not only in the chance of precipitation, but also temperatures 
and snow levels.  The NBM has slight chance to a chance (15-30%) of 
rain or rain/snow mix in the lower elevations and mountain snow.

Will it be a westerly flow or a split flow/trough on Thursday? There 
are large variations in the models.  NBM keeps a similar forecast as 
Wednesday and agree.  Wister/85


&&

.AVIATION...12Z TAFs...The only airport this morning reporting VLIFR 
is DLS, but this will improve slightly to IFR later this morning and 
VFR this afteroon.  All other terminals are VFR at this time, but 
there is a chance for MVFR or IFR as light rain develops in the 
afternoon.  Confidence was high enough to lower CIGS and/or VSBY to 
these flight rules at PDT, ALW and PSC.  Will keep VFR at YKM, RDM 
and BDN.  Winds will be 10 kts or less at all sites for the next 24 
hours.  Wister/85


&&

.PRELIMINARY POINT TEMPS/POPS...
PDT  52  38  47  34 /  50  40  30  20 
ALW  55  41  48  36 /  60  70  40  20 
PSC  52  39  50  36 /  40  30  20  10 
YKM  50  34  50  30 /  30  10   0   0 
HRI  51  37  48  34 /  40  20  20  10 
ELN  47  33  47  31 /  40  10   0   0 
RDM  52  32  45  26 /  10  10  10  10 
LGD  50  39  46  34 /  60  60  60  40 
GCD  49  33  46  30 /  70  20  30  30 
DLS  49  39  47  34 /  20  20  20  10 

&&

.PDT WATCHES/WARNINGS/ADVISORIES...
OR...None.
WA...None.
&&

$$

SHORT TERM...87
LONG TERM....85
AVIATION...85