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000
FXUS66 KMFR 240349
AFDMFR

Area Forecast Discussion
National Weather Service Medford OR
849 PM PDT Tue Apr 23 2024

.DISCUSSION...Thunderstorm activity is a few hours behind us now
and conditions will remain relatively calm overnight. Zonal large
scale flow will allow a weak boundary to drift in off the pacific
tomorrow. This will initiate another round of pop-up style
thunderstorms across NorCal and east of the Cascades. Storms are
not expected to be any stronger than today, though; with winds
generally being 40 mph or less and hail around pea size as a
result of these storms.

For areas that don`t receive thunderstorm activity, skies are
expected to still be partly cloudy as some higher level clouds
move through the area as we swap a warm airmass for a cool-ish
one. Temperatures may not even get into the 70s for most areas,
with the Lower Klamath river valley being the exception.

The pattern is expected to remain active into the weekend with
rain and snow becoming more widespread in the forecast.

&&

.AVIATION...24/00Z TAFs...VFR ceilings and visibilities are most
likely across the forecast area outside of lower stratus near North
Bend. Thunderstorms have been building over northern California with
ceilings around 6000 to 8000 feet above ground level. The storms
will impact smaller airports in northern California and will be in
the vicinity of Klamath Falls. The lower LIFR and IFR ceilings will
push back into the coast later tonight. -Smith/Hermansen

&&

.MARINE...Updated 800 PM Tuesday, April 23, 2024...Winds and seas
will continue to lower tonight into Wednesday. Then a front will
arrive on Thursday followed by additional fronts Saturday into early
next week. Westerly fresh swell and gusty southerly winds could
rebuild steep seas by late Thursday or early Friday, especially in
outer waters. However, current guidance suggests that waves only
have a 10-20% of exceeding 9 feet through the weekend. So while
steep seas and gusty winds up to small craft advisory level are
possible, more hazardous conditions are not expected.

-Hermansen/CC

&&

.PREV DISCUSSION... /Issued 252 PM PDT Tue Apr 23 2024/

SHORT TERM...Tonight through Wednesday night.

It will be at least a couple of weeks before it is quite as warm
again inland as it is today. This shift back to cool, frequently
wet, more typically spring-like weather will be more readily
apparent beginning on Thursday. But, the cooling trend will begin
Wednesday, with highs around 5 to 10 degrees lower than today.

The most notable aspects of the short term are areas of coastal
low clouds, and a chance of afternoon and evening showers and
thunderstorms for northern California, northward into the
Siskiyou Mountains of far southern Jackson County, and
northeastward into south central Oregon..Klamath and Lake
counties. This includes Fort Jones, Yreka, Weed, Mt. Shasta,
Tennant, Lava Beds, Alturas, and Lakeview.

Our area of activity will be a bit larger today with an area of
deformation and instability between areas of low pressure offshore
from Canada and California. The pattern will shift very slightly
southeastward for Wednesday with a late day slight chance to
chance of thunderstorms again for southern, central, and eastern
Siskiyou County eastward and northeastward across Modoc and
southern portions of Klamath and Lake counties. This includes
Tennant, Alturas, and Lakeview.

By the end of Wednesday night, a wet cold front is expected to be
just west of the coastal waters, with rain developing at the coast
Thursday morning.

-DW

LONG TERM...Thursday morning through Tuesday night.

The warm and dry weather comes to an end on Thursday as the next
wave and frontal system hits the region. We`re expecting rain
along the coast and west of the Cascades. The NBM is predicting a
100% chance of precipitation late Thursday morning through late
Thursday evening with chances of precipitation lowering to around
60% east of the Cascades. Snow levels will be around 5000 to 7000
feet depending where one is located with the higher snow levels
east of the Cascades. Like always, snow over the higher Cascades
is expected and Crater Lake will see accumulating snow fall
Thursday into Friday morning.

The forecast for Crater Lake is showing between 4 and 8 inches of
snow accumulating and perhaps 1 inch over Mt. Ashland. 1 to 3 inches
are also anticipated in the Warners through Friday morning. Overall,
impacts here are pretty minimal, although one traveling to Crater
Lake on Friday morning could be caught off guard with snow
accumulation in late April.

The atmosphere shows some instability by later Friday, so
thunderstorms were inserted into the forecast around that time. The
probability of thunder is around 15% and covers large swaths of the
forecast area. Given that an upper level wave is starting to depart
the region, thunderstorms that do form will likely be weak and a few
cloud to ground flashes seems the most likely scenario.

Another shortwave and warm front will push into the forecast area
around Saturday.  The chance of precipitation is only about 50% west
of the Cascades as the ensembles might have some issues with timing
or perhaps moisture content. Those probabilities drop off in
northern California and east of the Cascades down to 20 percent. So
some of the forecast area as a modest chance of rain, other areas
will likely stay dry.

Finally, the ECMWF and GFS are both showing a stronger low in the
Gulf of the Alaska, although the ensemble anomalies are not as
excited about how unusual it is for this time of year.  In general,
it looks like zonal flow is setting up under a strong west to east
mid level jet,  which will likely keep Spring like temperatures in
check with a cooler moist flow for the weekend into next week. There
is a small set of ensemble members(15%), which show a stronger ridge
building in southern Oregon between Sunday evening and Monday
evening, which would lead to warmer temperatures and drier weather
than what is currently in the forecast.

-Smith

AVIATION...23/18Z TAFs...VFR ceilings and visibilities are most
likely across the forecast area this afternoon and evening as lower
stratus has burnt off along the coast. Thunderstorms will build over
northern California this afternoon with ceilings around 6000 to 8000
feet above ground level. It also appears the storms will stay far
enough away from all our TAF locations, although the storms will
impact smaller airports in northern California. The lower LIFR and
IFR ceilings will push back into the coast later tonight.

-Smith

MARINE...Updated 115 PM Tuesday, April 23, 2024...The strongest
winds remain south of Cape Blanco but will be easing through this
afternoon. The thermal trough will weaken through the day, but a
Small Craft Advisory will be in place for all waters through 5 PM
today, then will continue for waters south of Cape Blanco until 8 PM.

Seas will remain calm through Wednesday and into Thursday morning
before active weather returns to the area. Westerly fresh swell and
gusty southerly winds could rebuild steep seas by late Thursday or
early Friday, especially in outer waters. However, current guidance
suggests that waves only have a 10-20% of exceeding 9 feet through
the weekend. So while steep seas are possible, more hazardous
conditions are not expected. -TAD/Hermansen

&&

.MFR WATCHES/WARNINGS/ADVISORIES...
OR...None.

CA...None.

PACIFIC COASTAL WATERS...None.

&&

$$