Area Forecast Discussion
Issued by NWS Medford, OR

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FXUS66 KMFR 171112
AFDMFR

Area Forecast Discussion
National Weather Service Medford OR
412 AM PDT Wed Aug 17 2022

.SYNOPSIS...Temperatures will peak today before following a
minimal downward trend into the weekend. Thunderstorm activity
will be possible over much of the area through Thursday, possibly
Friday, evening as monsoonal moisture is pulled into the region.
Fog, causing reduced visibility, and light drizzle will be
possible along the coast each night.

&&

.DISCUSSION...Water vapor and IR satellite imagery is showing
moisture returning from the south on the western periphery of
high pressure aloft over the Great Basin and low pressure
offshore of NorCal. This moisture is leading to cloud cover
across the east side and will continue to increase.

This cloud cover changes two things about the thunderstorm
forecast this afternoon. First, is that it lowers the chances
across the east side. Will maintain a slight 10-15% chance for
weak isolated activity. The second thing the cloud cover will do
is shift the better thunderstorm chances to the west. As the
afternoon sun gets the west side valley floors roasting, the
building instability will become ideal for convective initiation.

Convective Allowing Models generally show scattered showers with
isolated thunderstorm activity forming over the hot west side
valleys (**A Heat Advisory is out for the heat across most of the
area**). PW values should be well exceeding 1" and lapse rates
far above 7 deg-C/km. Because of the nebulous nature of the
monsoon moisture, the organizing wind shear is weak. Storms should
lumber from south to north with the large scale wind pattern.
This weak shear combines with instability driven mainly by surface
heating. This surface heating is great to get storms initiated,
but without colder air aloft the thermodynamic forcing is
resulting in low CAPE values.

These limited organization and low end instability values amongst
strong lapse rates suggest storms that will grow rapidly with
limited production of ice because the strongest storms shouldn`t
last much beyond an hour once they reach their mature phase. It`s
also worth noting the instability in the ice layer,
-10 to -30 degree C layer (the cloud tops), is limited to the
coldest part of this layer and is limited as well. A lack of
instability here has been associated with weak charge separation.

What should this mean for the non-meteorologist? Virga producing
showers will be scattered across much of the area with a 15-30%
chance of these becoming thunderstorms (west of the Cascades). The
thunderstorms should not linger much longer than an hour in their
mature phase before new storms develop. This shorter mature phase
and poor environment could make charge separation processes less
efficient, that would lead to lower cloud to ground lightning
counts (because intra and inter cloud lighting is easier to
achieve). Certainly cloud to ground lightning will be present
though and the dry surface layer combined with dry, receptive
fuels means that new lightning will likely result in new fire
starts. For this reason a Red Flag Warning is in effect.

The best thunderstorm environment begins shifting slightly east
on Thursday, generally from the Cascades east, while remaining
over most of NorCal. Models favor an environment more conducive to
organized storms, possibly moving out of Isolated and into
Scattered coverage amounts. Will be expressing the more favorable
thunderstorm conditions to the day shift.
-Miles

&&

.AVIATION...17/12Z TAFs...Along the coast, areas of LIFR to IFR from
Cape Blanco northward and into the Coquille Valley will persist into
the morning. Inland, VFR will persist, except in stronger
thunderstorms this afternoon and evening. Additionally, gusty and
erratic winds are likely with isolated to scattered thunderstorms.
-Sven/DW

&&

.MARINE...Updated 300 AM PDT Wednesday, 17 August, 2022...
A thermal  trough near shore will weaken with north winds gradually
decreasing into tonight. However, seas are expected to remain steep
through this evening. The lower winds will be short-lived, with
stronger north winds likely to return Friday into Saturday evening,
strongest south of Cape Blanco and during the afternoon and evening
hours. -DW

&&

.MFR WATCHES/WARNINGS/ADVISORIES...
OR...
CA...
Pacific Coastal Waters...
$$


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