NWS Medford, OR wrote its last area forecast discussion before the first event at Jun 20, 7:25 AM PST, 3 hours 35 minutes earlier.
Area forecast discussion, Jun 20, 7:25 AM PST, 3 hours 35 minutes before
Area Forecast Discussion
National Weather Service Medford OR
825 AM PDT Sun Jun 20 2021
.UPDATE...
We made a few changes to the forecast this morning as a few fires
have put out enough to smoke to add it into the forecast east of
the Cascades today. The most notable fire is the Cutoff fire near
Bly. Some of the smoke and haze will likely spill into valleys
west of the Cascades by Monday and Tuesday. The smoke will be low
concentration, so we're not expecting any health impacts to the
public, just some hazy skies. Otherwise, we added some stratus to
the southern coast near Brookings and North Bend this morning.
The stratus should clear out quickly this morning as the
boundary layer destabilizes.
Read the synopsis and discussion below for more information on
the upcoming heat and thunderstorms.
-Smith
&&
.SYNOPSIS...The upper level trough will weaken today allowing the
thermal trough to move east over the inland valleys. Near-record
highs are expected today and tomorrow for most of the interior (an
Excessive Heat Warning remains in effect, and details can be
found at PDXNPWMFR). Monday night as an upper low develops off the
California coast and the thermal trough weakens, temperatures
will begin to lower and the threat of thunderstorms increases.
Thunderstorms are a possible Monday evening through Wednesday
afternoon. By Thursday, upper level ridging rebuilds and a thermal
trough once again sets up along the coast as the low retrogrades
and rebuilds offshore of south central California. This can
potentially form a blocking pattern right along the West Coast and
cause temperatures to soar above and beyond this weekends heat.
&&
.DISCUSSION.../Issued 618 AM PDT Sun Jun 20 2021/
Took a look into the character of the thunderstorms
tonight for possible severe Monday and Tuesday afternoon. Also
looked into lightning potential, conditional to storms forming. A
summary is that storms Monday and Tuesday afternoon have a low
potential of being severe, calibrated guidance suggests there
could be frequent lightning with these storms, and that the storms
should produce rain that reaches the ground.
For the concern of severe, the area in question was for northern
Lake County near Crescent. This is an area that climatologically
favors large hail because of the boost from orographic lift. The
MUCAPE in that area has a 90% chance of exceeding 1000 J/kg, with
raw guidance suggesting the environment could exceed 1600 J/kg of
MUCAPE. The Effective bulk Wind Difference (the amount of shear
in the most unstable, inflow part of a storm) has a 70% chance to
reach or exceed 30 kt. And the large hail parameter for most
available deterministic is equal to or greater than 5 (where 4 is
the minimum threshold) where collocated with 700-500 mb lapse
rates greater than 7 C/km. These signs point to the possibility of
severe hail, that is hail greater than or equal to 1" (quarter
size). For these reasons, have added large hail wording to the
forecast, buffered by small hail wording, in the northern Klamath
and NW Lake County areas. However, after discussing with SPC a
few factors have formally kept a marginal convective outlook from
the forecast. First is that mid-level southerly flow isn't
forecast to strengthen until around 06z or later. This will
benefit the possibility of nocturnal storms, but with instability
fading overnight this creates too large of a timing difference to
ignore. Second, is that while CAMs do show storms firing in the
area of concern, there is enough variation in individual models
that we are not sold that they present as strong enough to be
severe. Third, is that there is a dry boundary layer, which will
melt hail and decrease its size when it reaches the surface. With
KMAX down during this time, will continue to monitor this threat
closely.
As for the lightning potential, which includes nocturnal storms.
SREF probability does indicate a 2.5-6% chance for greater than
or equal to 100 CG strikes Monday night, along the Cascade crest
area. Then there is another larger are of 2.5-6% chances of 100+
CG strikes Tuesday afternoon from roughly the NW Marble
mountains/western Siskiyou mountains NE along the Cascades and
spilling over into the east side of NW Lake County. Heading into
Tuesday evening/night this probability lowers to 2.5% and centers
over northern Lake County. This signal is present for multiple
runs and has good spatial continuity with each run.
Unfortunately, this piece of guidance is new to us, so we don't
have a great feel for its performance. So, here are some lighting
climatology stats to supplement this information. For this week
in June daily CG strikes in Klamath County are usually less than
or equal to 125 and rarely more than 310, but the daily max is
around 1000 strikes. These amounts are similar to the climo of
Siskiyou County and about 50 strikes low for Lake County. This
should help reinforce that the SREF probability forecast does seem
reasonable for this time of year given that storms form.
Another characteristic of these storms is that they should be wet
enough to produce rain at the surface. Median Precipitable Water
values are above 1" west of the Cascades and above 0.75" for the
remainder of the area. 700-500 mb moisture is generally above 70%
in deterministic solutions, which indicates that a good amount of
that moisture is at a height it can be ingested by developing
convection. This moisture combined with slow storm movement,
approximately 12-23 mph, should allow for precipitation to erode
the dry boundary layer, increasing the chance rain reaches the
surface. Under a slow moving thunderstorm, amounts could reach
1+". -Miles
&&
.AVIATION...
For the 20/12Z TAF Cycle...Gusty northerly winds are
expected again along the coast and in some north to south valleys
like the northern Shasta Valley near Yreka. IFR clouds/visibility
are impacting the coast and coastal valleys from Cape Blanco
northward. These lower conditions will likely improve late this
morning or early afternoon, only to return this evening. Elsewhere,
VFR conditions are expected through the next 24 hours. Keene
&&
.MARINE...
Updated 300 AM PDT Sunday, 20 June 2021...Gale force
winds will continue south of Cape Blanco through this evening with
winds at advisory strength north of Cape Blanco as a thermal trough
remains along the coast. Winds gradually diminish but seas remain
steep tonight into Monday as the thermal trough moves farther
inland. Light, near shore southerly winds develop south of Brookings
late tonight and spread to Cape Blanco by Monday evening. Seas are
expected to be around 5 to 7 feet late Monday into Tuesday.
Northerly winds increase again Wednesday afternoon, with gales
possible south of Cape Blanco Thursday into Saturday. Keene
&&
.FIRE WEATHER...Updated 800 PM PDT Sat 19 Jun 2021...Gusty west
winds and very low humidities east of the Cascades, with the lowest
humidities and gustiest winds in Modoc, are moderating at this time.
The Red Flag Warning for Modoc county will expire at 9 PM PDT this
evening.
An upper low will deepen off the California coast Sunday with south
winds setting in, and this will feed moisture into the area.
Instability will increase Monday with a scattered thunderstorms
possible in portions of the east side of Oregon. A Fire Weather
Watch has been issued for the area for Monday afternoon into the
evening. The possibility of thunderstorms will continue into Tuesday
morning. The upper low will move closer to the region just off the
coast of northern California Tuesday, and will continue to feed
moisture into the region. This will result in more thunderstorms
from Western Siskiyou county, and up the Cascades and across the
east side. There may be scattered thunderstorms in places, mainly
western Siskiyou county, the Cascades, and east of the Cascades.
This pattern remains into Wednesday before an upper ridge builds in
with near record temperatures possible next weekend.
&&
.MFR WATCHES/WARNINGS/ADVISORIES...
OR...Fire Weather Watch from Monday afternoon through Monday evening
for ORZ624-625.
Excessive Heat Warning from noon today to 11 PM PDT Monday for
ORZ024-026.
CA...Excessive Heat Warning from noon today to 11 PM PDT Monday for
CAZ080>082.
Pacific Coastal Waters...Small Craft Advisory until 5 PM PDT Monday for
PZZ350-356-370-376.
Gale Warning until 11 PM PDT this evening for PZZ356-376.