Scientific Forecaster Discussion

Return to Local Conditions & Forecast

Without Abbreviations
With Abbreviations

Area forecast discussion 
National Weather Service Seattle Washington
328 am PDT Sat Jul 23 2016

Synopsis...high pressure will bring dry and mild weather this
weekend into Monday. A weak system will move by to the north
Monday night bringing some marine air into the interior for
Tuesday. The ridge will strengthen Wednesday into the end of the
week with the warmest temperatures of the month expected.


Short term...hello Summer. Satellite imagery shows cloudy skies
over western Washington early this morning. Weak convergence in
the lower levels with katx Doppler radar indicating some light
showers over Snohomish County. With the cloud cover temperatures
were in a narrow range at 3 am/10z...mid 50s to lower 60s.

Convergence will dissipate this morning as the low level flow
becomes light. Upper level ridge building offshore with the 500 mb
heights over western Washington in the lower to mid 580 dms by
00z Sunday. Even with the high 500 mb heights...temperatures in
the lower levels remain a little cool with model 850 mb
temperatures only around plus 8c by 00z Sunday. The weakening of
the low level onshore flow combined with the strong July sun will
burn off the marine layer by afternoon but with the cooler
temperatures in the lower levels highs will be near to a couple of
degrees below normal the 60s along the coast and upper
60s to mid 70s inland.

Flat upper level ridge remaining over the area tonight into Sunday
with 500 mb heights remaining in the lower to mid 580 dms. Surface
gradients northwesterly onshore and light. Temperatures in the
lower levels warm on Sunday with model 850 mb temperatures in the
13-15c range. With less morning clouds and warmer temperatures in
the lower levels should see at least 5 degrees of warming on
Sunday. Locations in the interior from Seattle southward will be
in the lower 80s with 70s common over the north interior and
temperatures near 70 along the coast. Lows tonight will be in the
mid 50s to lower 60s.

Persistence a good forecast for Sunday night and Monday with
little change upper level pattern. Temperatures aloft warm
slightly on Monday but the low level onshore flow starts to
increase Monday afternoon leaving high temperatures similar to
sundays readings.

Long term...warmest days of July coming up the latter half of
next week. Extended models in good agreement with a very weak
shortwave moving by to the north Monday night. This will thicken
up the marine layer over western Washington a little Tuesday
morning and result in slightly cooler high temperatures...up to 5
degrees cooler versus Monday. Flat upper level ridge begins
strengthening Wednesday with 500 mb heights near 590 dms by
Friday. Surface gradients remaining northwesterly and onshore
through the period. This will put a cap on the high temperatures.
Highs on Wednesday will be in the 70s to mid 80s with about 5
degrees of warming on Thursday. Highs on Friday will be similar to
Thursday. The warmer locations Thursday and Friday will be near
90. There is a good chance Seattle gets to 85 Thursday and Friday
for the first time this month. Last year there were a record tying 12
days in July with highs 85 plus in Seattle. July 2014 had 11 days
with highs 85 plus. Felton


Aviation...a somewhat flat upper ridge will build in off the
Pacific late today and tonight. The low-level air mass will
remain moist this morning due to moderate onshore flow, with IFR
cigs over The Lowlands. Gradual drying is expected this PM due to
weak large-scale subsidence. Otherwise, the air mass will become
more stable over the next 24 hours. Light to moderate northwest flow

Ksea...moist low-level air mass will support cloud ceiling in the
010-018 range this morning. Lifting and scattering of the clouds
expected around mid-day, with clearing this evening. Light and
variable wind through this morning, then a north breeze will
gradually develop around 18z-20z. Haner


Marine...high pressure offshore and lower pressure east of the
Cascades will maintain onshore flow for the next week. The
strength of the onshore flow will mainly be moderate, but it
should become strong on Monday evening with westerly gales in the
Strait. Haner


Climate...tomorrow ( July 24 ) is the driest day in Seattle
weather records. Since records started in the 1890's there has
only been a total of 0.72 inches of rain recorded on the 24th.
There is only one other day in the year...July 30th with 0.75
inches of rain...where there has been less than an inch of rain.


Sew watches/warnings/advisories...
Pz...Small Craft Advisory until 3 am PDT Sunday for central U.S.
Waters Strait of Juan de Fuca-East Entrance U.S. Waters
Strait of Juan de Fuca.




You can see an illustrated version of this discussion at

National Weather Service Glossary of Abbreviations