Scientific Forecaster Discussion

Return to Local Conditions & Forecast

Without Abbreviations
With Abbreviations

Area Forecast Discussion
National Weather Service Spokane WA
225 PM PDT Thu Sep 29 2016

.SYNOPSIS...
There's an increased chance of showers and thunderstorms
overnight tonight. Noticeable cooling will begin on Friday with a
another chance of showers Friday night into Saturday morning. A
strong pattern shift to much cooler and unsettled conditions will
begin this weekend and continue into next week.

&&

.DISCUSSION...
Tonight through Saturday: An upper level low pressure system will
dig further offshore over the next 24 hours. Water vapor satellite
imagery shows a shortwave trough rounding the base of the upper
level low at 2:00 PM just off the northern California coastline.
It is progged to push up in the southwest flow pattern across the
region tonight. Models are all consistent with a ribbon of mid
level instability primarily east of line from Yakima to Republic
over to the ID Panhandle. The NAM model is much more unstable than
the GFS and ECMWF favoring the possibility for at least isolated
thunderstorm activity. I am not sold that the NAM is correct, but
we should at least see developing showers increasing after
midnight. This wave will exit the area tomorrow morning. Models
are showing a lot of dry air pushing in behind tonight's wave for
Friday, so we should a lull in precip for the afternoon before the
cold front arrives Friday night.

There are model differences with the amount of precip we will see
with the cold front. The 12Z GFS and NAM solutions are noticeably
drier than the 12 ECMWF solution. The ECMWF does not go as dry as
the GFS and NAM. Considering that the water vapor satellite shows
a considerable dry slot behind tonight's wave, I tend to believe
the drier solutions and will mostly discount the ECMWF at this
time. Best chances for precip will be over extreme eastern WA and
in the ID Panhandle. I do think that these areas will be a bit
wetter with the showers ahead of the cold front than with the
showers that develop tonight.

Winds will be a bit breezier and temperatures cooler on Saturday
in the post frontal air mass. Pressure gradients will not be
particularly strong and 850 mb winds only up to near 20 mph.
Expect sustained winds of up to 10-15 mph and gusts 20-25 mph for
more exposed areas to southwest winds. Temperatures Saturday will
feel much more like fall with highs in the 60s. /SVH

Saturday night through Monday...Enough model agreement exists to 
generate reasonable confidence that Sunday and Monday will be dry
over the forecast area as the upper low pressure system transits
through the Great basin keeping eastern Washington and north
Idaho under a quiet but cool col zone between the southern branch
and northern branch of a split flow regime. The only significant
concern during this period will be the potential for nippy morning
lows...possibly approaching or briefly achieving sub-freezing
temperatures after overnight clearing promotes radiational cooling
...not only in the wind protected northern valleys but also the
rising terrain of the eastern and northern and eastern Columbia
Basin. Some patchy fog will also be possible in the valleys which
received rain on Saturday. Otherwise mostly sunny conditions with
high temperatures a few degrees below normal.

Tuesday through Thursday...Confidence deteriorates dramatically
for this period regarding any timing details of migrating systems
capable of bringing showers or even periods of light stratiform
rain to any location in the forecast area except for the deep
basin and Cascades lee areas (they will probably be
characteristically dry). Both the latest GFS...EC and Canadian
models indicate a progressive and continued cool autumn pattern
impacting the region with periodic waves and disturbances capable
of generating precipitation outside of the normally risky
mountain locations...but the timing and character of these systems
varies significantly from model to model. The only near certainty
will be continued cooler than normal temperatures and unsettled
conditions through the end of next week. /Fugazzi

&&

.AVIATION...
18Z TAFS: Mid level instability will keep a deck of mid clouds 
and isolated showers over north Idaho. Another round of moisture
with elevated isolated showers will move into the region late this
evening and overnight...mainly east of KMWH to Colville. Cloud
bases will stay VFR but may get occasional clouds to near 4k ft
especially over north Idaho. Winds will remain light with
downslope winds increasing in KEAT tonight. /rfox. 


&&

.PRELIMINARY POINT TEMPS/POPS...
Spokane        51  72  46  62  43  63 /  30  20  70  20  10  10 
Coeur d'Alene  51  72  47  60  41  63 /  30  20  70  40  10  10 
Pullman        49  73  43  61  40  63 /  20  10  60  20  10  10 
Lewiston       56  78  51  67  45  68 /  20  10  40  30  10  10 
Colville       48  73  43  64  37  65 /  30  40  60  40  10  10 
Sandpoint      47  70  43  59  37  60 /  30  30  70  50  10  10 
Kellogg        49  72  44  57  37  60 /  30  20  60  60  20  10 
Moses Lake     48  75  41  68  41  68 /  20  20  10  10  10  10 
Wenatchee      51  73  46  66  42  66 /  10  10  10  20  10  10 
Omak           50  74  43  67  39  66 /  10  20  10  20  20  10 

&&

.OTX WATCHES/WARNINGS/ADVISORIES...
ID...None.
WA...None.
&&

$$

National Weather Service Glossary of Abbreviations