Frost and Freeze in Oregon, March 31, 2021

Area Forecast Discussion
National Weather Service Medford OR
1040 PM PDT Tue Mar 30 2021

.Updated aviation discussion.

.DISCUSSION...An update to the forecast is not necessary. A ridge
with its axis offshore, along 130W will build inland on Wednesday.
This will continue clear skies with another morning of cold
temperatures (except at Brookings), and afternoon of very warm
temperatures inland. As a result, a Freeze Warning has been issued
for the valleys of Josephine and Jackson counties.

Brookings will be the exception with breezy easterly winds to
continue overnight, then weaken in the morning. This will result
in a morning low in the upper 40s, then a high near 65 to 70 with
a maximum likely in the late morning before onshore winds return.

The 18Z data supports a forecast of dry weather into Saturday
night. Model solutions diverge more significantly thereafter. But,
a shift to a cooler, showery pattern is likely.

&&

.AVIATION...For the 31/06Z TAF Cycle... VFR conditions will prevail
through the TAF period under a warm and very dry air mass. Gusty
winds will continue just offshore this evening. Inland, easterly
winds will be gusty over the ridge tops into Wednesday morning,
especially over the Coast Range and the Cascades. -DW/Keene

&&

.MARINE...Updated 800 PM PDT Tuesday 30 Mar 2021....North winds will
gradually diminish overnight, but seas will remain very steep south
of Cape Blanco beyond 10 NM from shore, with steep seas most other
areas. Conditions improve Wednesday afternoon with lighter north
winds and seas generally at or below 6 ft through at least Friday
morning. Seas may increase some over the weekend but will probably
remain below Small Craft Advisory criteria through Saturday.
Miles/Keene

&&

.PREV DISCUSSION... /Issued 303 PM PDT Tue Mar 30 2021/

SHORT TERM...

The quiet weather pattern continues with high pressure and dry
air lingering over the Pacific Northwest. In addition, a thermal
trough will build up towards Seattle Tuesday night into Wednesday
morning resulting in a very dry airmass over most of Oregon. This
dry air will help temperatures drop very quickly overnight. We're
forecasting low temperatures around 28 to 32 degrees within many
valleys west of the Cascades. Along the coast, temperatures will
be rather warm near Brookings with lows in the mid 40's. However,
temperatures in North Bend and most of the Central Oregon
coastline will fall into the upper 30's tonight.

Because of the cold temperatures within many valleys west of the
Cascades, we're going to issue a freeze warning for the Illinois
Valley, Applegate Valley and Rogue Valley. The probability of sub
freezing temperatures is around 40 to 50 percent in many valleys
according to the NBM. The NBM has been known to struggle with
overnight lows in the valleys during clear nights as the lows end
up cooler than expected. Therefore, we feel confident most
locations in the valleys mentioned above will reach freezing
tonight based on the information above.

It should also be noted that the inversion is expected
to be quite strong tonight. Deterministic model soundings are
showing temperatures in the mid 40's and 50's over the mid slopes
and ridges tonight.

By Wednesday, high pressure will continue to build with 500 mb
heights continuing to build over the region. Temperatures will be
in the 97th to 99th percentile west of the Cascades on Wednesday
as highs test the 80 degree mark in Medford. Temperatures this
warm will be around 15 to 20 degrees than normal for this time of
year. We're currently forecasting near record temperatures for
Montague, CA. However, our other climate locations are about 3 to
5 degrees too cool to break their respective records for the
3/31.

By Thursday, cooler marine air will push into southern Oregon and
northern California. Temperatures will trend cooler along the
coast and west of the Cascades. However, east of the Cascades,
temperatures will remain fairly similar to what will occur on
Wednesday.

-Smith

LONG TERM...Fri, April 2nd through Tue, April 6th...
This long term period will begin with a broad southwest flow over
the forecast area on the western side of an upper level high
pressure ridge and east of an approaching upper level trough. High
temperatures are expected to be 10 to 15 degrees above normal for
areas inland from the coast. Normal highs are in the mid 50s in the
Klamath Falls area and around 60 in the Rogue Valley for this time
period. Partly cloudy skies are expected. Lows Friday morning will
be within 5 degrees of normal due to a dry air mass in place, which
means it will be below freezing east of the Cascades and some frost
will be possible west of the Cascades.

High temperatures are then expected to fall off by about 5 degrees
each day through Sunday as the upper level trough approaches, while
lows are not expected to change much. There are some definite timing
differences among the ensemble members regarding how fast the upper
level trough will arrive and move through. The majority of ensemble
members move the upper level trough through Sunday or Monday, with
the 06/12Z GFS deterministic run being one of the fastest solutions
and the 06/12Z ECMWF deterministic run being one of the slower
solutions. The slower solution indicated by the ECMWF would yield
more precipitation than would the faster GFS solution. At this point
we can be reasonably confident that precipitation will arrive for
Oregon west wide areas Sunday into Monday, but we're less certain
for areas south and west. Also, temperatures will dip to closer to
normal as will snow levels.

Ensembles also suggest that showery, seasonable April weather is
most likely for next week as upper level troughing generally
lingers. ~BTL

&&

.MFR WATCHES/WARNINGS/ADVISORIES...
OR...Freeze Warning from 2 AM to 8 AM PDT Wednesday for ORZ024-026.

CA...None.

Pacific Coastal Waters...Small Craft Advisory until 11 AM PDT
     Wednesday for PZZ350-356-370-376.
     Hazardous Seas Warning until 11 PM PDT this evening for
     PZZ350-356-370.
     Hazardous Seas Warning until 11 AM PDT Wednesday for PZZ376.

DW/Keene/Smith