Flash Flood in Washington, August 11 to 12, 2024

Area Forecast Discussion
National Weather Service Seattle WA
303 PM PDT Sun Aug 11 2024

.SYNOPSIS...Upper level troughing over the region will promote
onshore flow and cooler temperatures the next several days.
Isolated thunderstorms will be possible across the North Cascades
this evening and again Monday. More widespread chances of
precipitation may return to western Washington by next weekend.

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.SHORT TERM /TONIGHT THROUGH WEDNESDAY/...Current satellite
imagery does show stratus still draped across portions of the
interior this afternoon. Stratus has been slow to scatter, but
will continue to do so through the afternoon hours to make way
for some spots of sun across the interior lowlands. Stratus does
not look to clear near the coast, however. Temperatures this
afternoon have been trending in the 60s to near 70 across the
majority of the region and look to top out in the mid 60s to low
70s for most spots. Slight chances of thunderstorms remain
possible across portions of the North Cascades this afternoon and
evening.

Upper level troughing will persist over the area through the
first half of the week, generally promoting cooler and cloudier
conditions across western Washington. Expect additional rounds of
marine stratus to push inland during the overnight and early
morning hours the next few days. Afternoon high temperatures will
generally top out in the 60s and 70s. Some isolated thunderstorm
chances will still be possible for portions of the North Cascades
on Monday afternoon and evening, with probabilistic guidance
indicating a 10-15% chance of thunderstorm development across the
far northern areas near the Cascade crest and border. Chances
across the Cascades really wane heading into Tuesday as the
threat shifts further off to the east.

.LONG TERM /THURSDAY THROUGH SUNDAY/...Upper level troughing
will remain the dominant influence across the Pacific Northwest
through the long term period. Another low dropping down into the
region from Canada late in the week could bring more widespread
shower chances back to western Washington by the weekend, however
will need to see how guidance continues to trend with this over
the coming days. High temperatures on Thursday look to trend
closer to normal for right now (in the mid to upper 70s), with
temperatures dipping to below normal (upper 60s to low 70s) again
Friday into the weekend.

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.AVIATION...West to southwest flow aloft with upper level troughing
over western Washington. Marine stratus has been very slow to
evaporate today with MVFR to IFR conditions continuing at all the
area terminals. Satellite shows evaporation progressing northward
from the south, but has not budged through the North Interior. At
this rate, is it a possibility that some areas may not clear out at
all today. However, expecting slow ceiling improvement in the
interior continuing through 00Z Monday and into the early evening
for a brief return to VFR conditions. The is not expected to clear
out tonight. Stratus will redevelop again tonight for a return to
IFR and locally lower ceilings through Monday morning. Improvement
again tomorrow looks to be slow and ceilings linger into the
afternoon. Winds will remain west-southwesterly 5 to 10 kt
throughout the TAF period.

KSEA...Conditions are now VFR at the terminal with marine stratus
slowly evaporating from south to north as it lingers near the area.
VFR conditions should remain through this afternoon and evening.
Marine stratus and IFR ceilings look to return overnight and remain
through the morning, slowly lifting and evaporating by the afternoon
Monday. Winds remain southwesterly 5 to 10 kt.

LH

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.MARINE...Broad high pressure over the East Pacific with low
pressure inland will maintain northwesterly onshore flow over the
area waters. Diurnally driven pushes down the Strait of Juan de Fuca
will occur each evening. Currently winds look to remain below any
thresholds through the upcoming week.

Seas 6 to 8 ft at 8 seconds will ease tonight, which will allow the
Small Craft Advisory to expire later this afternoon. Seas subside
Monday back to 3 to 5 ft and remain like so through the week.

LH

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.FIRE WEATHER...Elevated fire weather conditions will persist
through this evening across portions of the North Cascades as
another shortwave trough brings the potential for thunderstorm
development. Coverage across the western slopes looks to be less
than that of the previous days, however any thunderstorms that do
develop may produce frequent lightning, brief heavy downpours,
hail, and gusty and erratic winds. Isolated thunderstorms will be
possible across the North Cascades on Monday afternoon and evening
again, however probabilities continue to wane as the greatest
threat moves off to the east.

Broader troughing setting up over the region this week will promote
increased onshore flow and good relative humidities across the region.
Apart from the isolated thunderstorm threat in the far northern Cascades
on Monday, this overall trend will help to alleviate fire weather conditions
through at least midweek. 14

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.SEW WATCHES/WARNINGS/ADVISORIES...
WA...None.
PZ...Small Craft Advisory until 5 PM PDT this afternoon for Coastal
     Waters From Cape Flattery To James Island 10 To 60 Nm-
     Coastal Waters From James Island To Point Grenville 10 To
     60 Nm.

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