Area Forecast Discussion
Issued by NWS Spokane, WA

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FXUS66 KOTX 260940
AFDOTX

Area Forecast Discussion
National Weather Service Spokane WA
240 AM PDT TUE MAY 26 2015

.SYNOPSIS...
A slow moving low pressure system will bring scattered afternoon
and evening showers and thunderstorms to the Inland Northwest
through Wednesday. Locally heavy downpours, frequent lightning,
and small hail will be possible the next several days. Low
pressure will gradually depart by Thursday, bringing a decrease in
precipitation chances, but chances may increase again by early
next week across the region. Temperatures warm back above normal
from Wednesday into the end of the week, with highs pushing into
80s Thursday onward.


&&

.DISCUSSION...
...SLOW MOVING THUNDERSTORMS TO PRODUCE PERIODS OF VERY HEAVY RAIN
AT TIMES TODAY...

Today and Wednesday: Satellite imagery at 2:00 AM shows the upper
level low pressure system centered right over Deer Park. Much of
the region east of the Cascade Mtns has at least some weak mid
level instability that is fueling the overnight shower activity.
This instability is a little bit better over the southeast portion
of the region with some isolated thunderstorms popping up from the
WA Palouse over the Central Panhandle Mtns and points southward. I
expect a chance for thunderstorms through the rest of the early
morning hours over these same areas.

The focus will then shift a bit to a deep fetch of moisture
wrapping around the upper level low. Water vapor satellite shows
good enhancement extending from southwest Montana up into southern
BC. The trend through the rest of this morning is for this
moisture to wrap around into north central WA. The NAM model
solution shows a strip of moderate to heavy rainfall extending
across southern BC and curling into the east slopes of the
northern Cascades with this fetch of moisture. It would indicate
widespread rainfall amounts of around a quarter to a half of an
inch over a 3 hour period. It is conceivable that localized higher
rainfall rates are possible due to steep mid level lapse rates
resulting in convective enhancement. Higher resolution models
would agree with the potential for some heavy rainfall entering
the northern Cascades as the HRRR, WRF-NMM and WRF-ARW models
agree with this potential. The GFS and ECMWF show lighter rainfall
amounts, but it is difficult to discount the higher resolution
model depiction for this morning. This will be a concern for
potential flooding related impacts; most notably debris flows or
mud slides in steep terrain or on recent burn scars such as on the
Carlton Complex. Confidence is not high enough at this time to
issue a highlight, but this situation will definitely be monitored
through the morning hours.

Much of the region will see an increasing shower and thunderstorm
threat late this morning into the afternoon. The focus will once
again reside mostly across the northern portion of the forecast
area and over the Cascade Mtns where the higher moisture flux will
be. The potential for flash flooding and debris flows will
increase as this convection gets going. This will be especially so
over the mountains in steep terrain and over recent burn scars
from the 2013 and 2014 fire seasons. Not only are thunderstorms
expected to produce periods of very heavy rainfall, but storm
motion will be slow.

The upper level low will begin to pull away to the southeast
Tuesday night. This will result in a decreasing shower trend into
Wednesday, but much of the higher elevations will see a good
chance for convection in the afternoon as the atmosphere remains
unstable. The flash flood threat will be lower on Wednesday
compared to today. Temperatures will rebound on Wednesday as well,
especially over the western portion of the region where
temperatures in the mid 70s and 80s are expected to return. /SVH

Wednesday night through Sunday...One upper level moves out of
the region on Wednesday with another poised to drop into B.C. and
track east north of the border Wednesday through Saturday or early
Sunday. Meanwhile another deep low pressure system will spin out
near 150w without much movement. This will put the region in a
northwest-northeast flow Wednesday night and Thursday before
becoming mainly southwest to westerly through the weekend.
Numerous weak waves will move through this flow pattern. Model
guidance is pretty consistent showing afternoon and evening
instability due to surface heating just about every day. Combine
this with these waves and late day showers and thunderstorms have
been kept in the forecast mainly tied to the higher terrain
outside of the basin. Hard to pin down specifics but the
Palouse...West Plains and the Spokane-Coeur D`Alene corridor at
times. Normally there would be a bit of drying with the this
pattern change but the models are hanging on to pretty deep
moisture with PWATs remaining well above normal. There is not
really anything to push this convection so the biggest concern
will be slow moving thunderstorms dropping a lot of rain in a
short period time, so expect a good chance for additional flood
advisories through the week. Temperatures will increase into the
low 80s to low 90s through Sunday which will be well above normal
for this time of the year.

Sunday night through Tuesday the low that has been hanging out in
the eastern Pacific will begin to move east towards the coast. The
models, as usual are having some timing differences but just about
all the Ensemble means show a low moving from off the OR/CAL coast
and track it northeast through the PAC/NW through this time
period. This is another good and wet shower/thunderstorm pattern
for the region. Slightly higher pops and cooler temperatures were
put into the forecast for both Monday and Tuesday. Tobin

&&

.AVIATION...
00Z TAFS: There is good model agreement that a closed low over the
Cascades early this evening will move into the Spokane/Coeur
D`Alene area overnight and remained parked over this area through
Tuesday. This will result in a continuation of showers and
thunderstorms over the area...with the highest coverage during
peak heating (afternoon/evening). Although tonight there will
likely be more of a focus of the showers around and north of the
KGEG/KSFF/KCOE TAF sites with the closed low tracking into the
area. Brief heavy rain and small hail has been observed with these
storms.  JW


&&

.PRELIMINARY POINT TEMPS/POPS...
Spokane        68  51  75  54  82  57 /  70  30  30  20  10  20
Coeur d`Alene  67  48  75  51  81  53 /  70  40  40  20  20  20
Pullman        68  48  71  49  77  51 /  60  40  50  20  10  20
Lewiston       73  54  76  55  84  57 /  50  40  50  20  10  10
Colville       71  49  80  51  85  54 /  70  40  30  20  20  20
Sandpoint      67  48  75  49  79  52 /  70  50  50  30  40  40
Kellogg        65  46  71  48  78  49 /  70  50  70  30  40  40
Moses Lake     76  53  82  56  89  57 /  60  30  10  10  10  10
Wenatchee      74  56  82  61  88  63 /  80  40  20  20  10  10
Omak           73  51  83  53  87  56 /  80  40  30  20  10  10

&&

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

$$


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