Scientific Forecaster Discussion

Return to Local Conditions & Forecast

Without Abbreviations
With Abbreviations

fxus66 ksew 281039 

Area forecast discussion
National Weather Service Seattle Washington
300 am PST Tue Feb 28 2017

weak higher pressure aloft should provide a brief break in the wet
weather pattern today. The next weather system arrives tonight with
some rain in The Lowlands and snow in the mountains as temperatures
moderate. Another stronger wetter system is expected by late
Thursday with more rain and mountain snow into Friday, and
temperatures close to normal. Lower pressure aloft is forecast to
develop off the coast this weekend for cooler than early March
temperatures and wet conditions with snow levels below the passes.


Short term...
the cool upper trough that produced plenty of wintry showers Monday
has moved inland. Showers tapered off overnight with a few lingering
showers remaining early this morning. Temperatures will around the
freezing mark in many areas. Given the wet pavement, icy spots are

The overall pattern looks to finally make some adjustments this
week. Progs agree on backing the flow from northwest today to more
westerly through Thursday. The change will permit some temperature
moderation over the next few days.

A weak shortwave ridge today before another shortwave in the Gulf of
Alaska this morning will swing onshore late today and tonight for
some relatively light precipitation. Snow levels should climb above
The Lowlands but still remain below the passes. Enough quantitative precipitation forecast with a
more orographic pattern for a Winter Weather Advisory in the
Cascades tonight into Wed morning. Low level onshore flow increases
through the day today for some blustery conditions.

Wednesday should offer a bit of a break in the weather with some
lingering showers before the next stronger Pacific system arrives
Thursday and Thursday night. The moderating trend continues with
this system with high temperatures rising close to early March
averages. More rainfall with snow in the mountains expected...
suggesting March coming in like a lion. Buehner

Long term...models are in good agreement that the end of the week
will be an active period with some generous snowfall in the
mountains. Pops and quantitative precipitation forecast were boosted from previous forecasts for
Friday through the weekend. Heights will take a tumble on Friday as
a vigorous front moves through western Washington. A rather deep
upper level trough will take up residence over the offshore waters
for the weekend with a series of disturbances rotating into western
Washington. The uw WRF-GFS shows some rather large swaths of the
Cascades and Olympics picking up a couple feet of snow in the 48
hour period ending Saturday afternoon.

Both the GFS and Euro drop the 1000/850 millibar thickness to sub
1300m levels with 850 temps of -6 or -7 c by the early hours of
Sunday. But with a southerly gradient prevailing, this should keep
snow levels at 500 to 1000 feet. The air mass begins to moderate
somewhat on Monday, but the pattern remains active with snow levels
at or below the passes. 27

&& upper trough over the interior Pacific northwest will
continue shifting eastward overnight. Light northwest flow aloft
through Tuesday. Air mass moist and slightly unstable but
thunderstorms are unlikely. Rain and snow showers will decrease
Tuesday morning, then a warm front approaching the area will bring
light rain to the area by Tuesday evening.

Rain and snow showers tonight will decrease Tuesday morning with
snow levels near 500 feet. However, heavier showers and any
convergence zone activity will temporarily drive the snow level down
to near sea-level at times through early Tuesday morning. 1 to 3
inches of snow are possible overnight along the Strait and in parts
of central/north greater Puget Sound, especially above a few hundred
feet elevation. Terminals that have the best chance of some snow
accumulation include sea/pae and clm. Temperatures will be near or
just above freezing through Tuesday morning at most terminals. The
Puget Sound convergence zone will also cause short periods of IFR
cigs/vis with snow and upslope snow along the Strait may produce
some IFR through around midnight. Elsewhere, mostly MVFR with VFR
stratus by midday Tuesday. Light rain develops Tuesday evening with
another slightly warmer system approaching.

Ksea...the convergence zone remains active and will meander near the
terminal much of the night. Winds will be fairly light (up to 6 kt)
and variable or easterly. Precipitation bands may be moderate to
briefly heavy with mixed rain/snow showers going over to snow. A
quick inch or two is possible through 12z (4am tue) before showers
taper off. Cigs mostly MVFR but IFR will be possible during heavier
showers or snow. VFR stratus likely by midday Tuesday. Brief drying
Tuesday afternoon then more rain arrives Tuesday evening. Dtm


Marine...a weak surface low over wrn WA tonight will dissipate by
Tuesday morning. Small craft northwesterly winds at times across the
coastal waters with a lull midday Tuesday before picking up again
late in the afternoon and evening. Seas near 10 feet including Grays
Harbor where rough bar conditions are expected through Tuesday
evening, especially around the 530 am and 545 PM Ebb currents
Tuesday. Small craft westerlies could develop Tuesday in the Strait
by late morning or afternoon. Another front may bring small craft
winds to some interior waters by Tuesday night and Wednesday. Dtm

&& flooding is expected through this weekend.


Sew watches/warnings/advisories...

Pz...Small Craft Advisory coastal waters and for Grays Harbor bar.



National Weather Service Glossary of Abbreviations