Hail in Oregon, August 3, 2023

Area Forecast Discussion...Updated
National Weather Service Pendleton OR
952 AM PDT Thu Aug 3 2023

.UPDATE...The flow over the region has become more southerly
overnight than on Wednesday. This is pulling monsoonal moisture
and instability northward into southern areas of the forecast
area. This moisture combined with instability will lead to
afternoon and evening thunderstorms today, and continuing on
Friday. Some of the storms will have very little to any rainfall.
As a result, these are where the best chances for new fire starts
will be due to lightning. Under the thunderstorm precipitation
cores, there may be ignitions that become holdover fires, which
will become active with rapid spread when it turns hot and dry
again. Afternoon minimum RHs today will be from 15-25 percent,
with the highest RHs along the Cascade east slopes. The valleys
adjacent to the east slopes will have the lowest RHs, in the
teens. RHs will also be higher in the mountains in the east and
northeast. The greatest concentration of thunderstorms today will
be from eastern Crook county eastward over Grant County and
northeast to Wallowa County. There are Red Flag Warnings for all
of the southern and northeast fire zones for thunderstorms with
abundant lightning. See the Fire Weather discussion for more
details about the Red Flag Warnings that are in effect.

Thunderstorms will persist on Friday, with about a 30-40% chance
of nocturnal storms overnight tonight due to elevated
instability. Friday's storms will be most concentrated over Grant
county, north-northwest to the Lower Columbia Basin and eastward
to the Idaho border. These storms on Friday will be wetter, with a
much better chance (60-80% chance) of receiving wetting rains.
The amount of shear will be low and the upper steering winds will
be weak, so thunderstorms will be mostly pulse type storm that
will be slow moving. There will be a continued chance of storms
over the CWA on Saturday, but they will be wetter, with less
coverage.

High temperatures will be a couple degrees cooler today, but still
near to slightly above normal. Highs will be in the lower to mid
90s in the lower elevations, and 70s to mid 80s mountains. Friday
will be considerably cooler with highs in the mid 80s to near 90
lower elevations, and upper 60s to near 80 mountains. Current
forecast looks good. 88

&&

.FIRE WEATHER....As described in the short term forecast
discussion above, there is a high confidence (80-90%) for
isolated to scattered thunderstorms with abundant lightning over
the central and eastern Oregon mountains this afternoon through
Friday evening. A Red Flag Warning for fire zones OR611, OR640,
OR642, OR644 and OR645 continues to look warranted and will remain
in effect from this afternoon through Friday evening. There will
be enough instability and monsoonal moisture moving into central
OR eastward and northeast to the NE mountains and Wallowa county
for isolated to scattered thunderstorms. At this time, the
greatest chances for LAL's of 3 or greater will be along the
southern and eastern borders of the district. Measurable
precipitation chances will be 20-40% with thunderstorms today.
Some storms may contain little rainfall under the storm cores,
with little to no  surrounding the cores, which is
where the great chances for new fire starts occur due to
lightning.

It will also be locally breezy to windy in the eastern Columbia
River Gorge and Kittitas Valley this afternoon and evening, but
the area of critical RH and critical wind overlap is small and
only for a couple hours. Therefore will headline the fire weather
products with elevated fire conditions there, but Red Flag
highlights are not warranted due to the short duration and small
size of the areas of critical wind and critical RH's overlap that
would cause Red Flag conditions.

&&

.AVIATION...18Z TAFs...VFR conditions will continue through the
period. Mid/high level clouds will increase through the period.
Wildfire smoke will continue to affect portions of Central Oregon,
but at this time not expected to impact KRDM/KBDN. Winds will be 5-
15 mph with higher afternoon gusts. Winds will decrease to 10 kts or
less overnight.  Earle

&&

.PREV DISCUSSION... /issued 431 AM PDT Thu Aug 3 2023/

SHORT TERM...Today through Saturday Night...A large upper trough
will remain off the Pacific Northwest coast with a weak low
circulation over our area through the next few days. A southwest
flow from the ridge along with circulation around the low will
pull monsoonal moisture into our area. This moisture along with
instability due to the low will give portions of our area isolated
to scattered thunderstorms mainly in the afternoon and early
evening each day though some activity is expected through the
nights. Confidence is good as the NBM Thunderstorm Probabilities
are 20-50 percent from central Oregon through the Ochoco-John Day
Highlands and the southern Blue Mountains eastward to the Wallowa
Mountains today and Friday. On Saturday, the NBM has a 20-50
percent chance of thunderstorms over the eastern Oregon mountains
and also in the Washington Cascades and Kittitas Valley.
Thunderstorms are not expected to be particularly strong but being
high based storms, rain will mostly evaporate, at least
initially, so rain amounts will be light for the most part, though
the forecast does have a couple of tenths of an inch around
Seneca this afternoon where thunderstorms are expected to be most
numerous.

Today will see a southwest flow over the area pulling in moisture
while a low will be to our east though the GFS has a secondary
low near Seneca. This will allow thunderstorms to develop this
afternoon with scattered storms over the Ochoco-John Day Highlands
and the John Day Valley while a slight chance of thunderstorms
will occur from east of Bend into the southern Blue Mountains and
then into the Grande Ronde Valley. This is the area covered by a
Red Flag Warning for abundant lightning and the RFW continues to
look warranted. The rest of the area will be dry but will have
increasing cloudiness in the late afternoon and evening.
Temperatures will be slightly cooler than yesterday but still in
the upper 80s to mid 90s in the lower elevations and in the mid
80s to mid 90s in the mountains. Pressure gradients will remain
tight enough for a weak marine push through the Cascade gaps and
the Kittitas Valley and the Columbia Gorge will reach 15 to 20 mph
though humidity will have risen to the low 20s. There could be
some locally critical fire danger issues for an hour or two in
those areas, but the threat is not sufficient for a Red Flag
Warning there.

Tonight, the showers and thunderstorms will push to the northeast
and have kept a slight chance of thunderstorms over the eastern
mountains and Blue Mountain Foothills along with a slight chance
to chance of showers. Do not expect more than a few thunderstorms
overnight, but couldn't rule them out completely. The cloud cover
will keep temperatures up with lows in the 60s with 50s in
central Oregon and the mountains.

On Friday, models have the weak low circulation directly overhead
and this will give us a slight chance to chance of showers over
the eastern mountains and most of the Columbia Basin along with a
slight chance of thunderstorms. Coverage in the evening will
shrink back to mainly the Ochoco-John Day Highlands. Temperatures
will drop 2 to 5 degrees. Winds will be lighter than today.
Saturday will see the upper low still over our area but a little
further north and it will tap into more monsoon moisture over the
Rockies. This will give us a chance of thunderstorms in the
afternoon and evening, mainly over the higher mountains of
eastern Oregon and the Washington Cascades. It will taper off to
showers overnight. Saturday will be a couple of degrees warmer
with highs in the upper 80s to lower 90s with mid 70s to mid 80s
in the mountains. Perry/83

LONG TERM...Sunday through Thursday...Overall, ensemble models show
good agreement through the extended period while deterministic
models shows some issues in timing and amplitude of shortwaves as
would be expected.

Sunday will see a shortwave undercutting the overall upper level
ridge over the western USA. This shortwave will interact with the
lingering moisture from the monsoon surge to produce some showers
and possible thunderstorms for most of the forecast area except
central Oregon where westerly flow will lead to downsloping along
the east slopes of the Cascades. This shortwave exits to the east
Sunday evening as another shortwave in the eastern Pacific upper
level trough prepares to move into the region on Monday afternoon
and night. This will be a fast moving shortwave that pushes a cold
front through the region late Monday. The front will trigger some
showers and possible thunderstorms as it moves across the forecast
area Monday afternoon through early Tuesday but also increase
westerly behind it that continue through Tuesday. At this point RH
values do not look to be critically low but will be worth watching
closely as the event approaches. The exiting of the front on Tuesday
will be followed by a cooler and dry westerly flow through Wednesday
before starting to warm up on Thursday.

FIRE WEATHER....As described in the short term forecast
discussion above, there is a high confidence (80-90%) for
isolated to scattered thunderstorms with abundant lightning over
the eastern Oregon mountains this afternoon through Friday
evening. A Red Flag Warning for fire zones OR611, OR640, OR642,
OR644 and OR645 continues to look warranted and will remain in
effect from this afternoon through Friday evening. There will be
enough instability and monsoonal moisture moving into central OR
eastward and northeast to the NE mountains and Wallowa county for
isolated to scattered thunderstorms. At this time, the greatest
chances for LAL's of 3 or greater will be along the southern and
eastern borders of the district. It is around the outer edges of
the precipitation cores, where there is the best chance for
lightning that causes new fire starts, and holdover fires are
likely where there is rainfall. Measurable precipitation chances
will be 20-40%, but this will be mainly under the thunderstorm
cores.

It will also be locally breezy to windy in the eastern Columbia
River Gorge and Kittitas Valley this afternoon and evening, but
the area of critical RH and critical wind overlap is small and
only for a couple hours. Therefore will headline the elevated fire
conditions there, and not issue any Red Flag Warnings for the
eastern Gorge due to the short duration and small size area of
these elevated conditions. Also, RHs are expected to increase to
above 20%, causing a decrease in fire conditions tonight as
moisture increases with the forecast weather pattern. Perry/83

&&

.PRELIMINARY POINT TEMPS/POPS...
PDT  89  63  85  60 /  10  20  30  10
ALW  94  67  88  64 /  10  20  20  10
PSC  95  68  91  68 /   0  10  20  10
YKM  93  63  90  63 /   0   0  10  10
HRI  94  67  90  65 /  10  10  20   0
ELN  90  61  90  61 /   0   0   0  10
RDM  89  55  87  54 /  10  10  20  10
LGD  91  61  84  59 /  10  40  30  20
GCD  90  58  86  55 /  20  40  40  20
DLS  93  64  93  65 /   0  10  10   0

&&

.PDT WATCHES/WARNINGS/ADVISORIES...
OR...Red Flag Warning from 11 AM this morning to 11 PM PDT Friday for
     ORZ611-640-642-644-645.

WA...None.

&&

UPDATE...88
SHORT TERM...83
LONG TERM....91
AVIATION...81
FIRE WEATHER...88