Scientific Forecaster Discussion

Return to Local Conditions & Forecast

Without Abbreviations
With Abbreviations

fxus66 kmfr 232127 

Area forecast discussion
National Weather Service Medford or
227 PM PDT sun Jul 23 2017

Discussion...the latest water vapor image clearly shows the upper
low off the California coast which has been advertised to happen the
last couple of days. The upper low is already tapping into some
monsoonal moisture and the result is mid level altocumulus clouds
that are moving from south to north into our area. Typically
altocumulus clouds are a indicator of increasing instability and
moisture and in some cases could be a precursor to thunderstorm
activity. Precipitable waters are higher with this mornings sounding showing 1.02

As previously mentioned in earlier discussions, the short range high
res hrrr model continues to show first storm development showing up
in Alturas County later this afternoon and migrating north into lake
and southeast Klamath County early this evening. This has been
consistent with the last several model runs (every hour).
Instability will be greatest over the northern Cascades this
afternoon. 700mb moisture is not as impressive, but precipitable waters are
expected to increase this afternoon, so still could not rule out
isolated storms late this afternoon.

The upper flow becoming southeast and this in combination with
increasing instability and hotter temperatures will bring an
increased risk for at least isolated thunderstorms mid to late
Sunday afternoon through Sunday evening. Right now, we think the
storms will first develop over the Cascades, Siskiyous and portions
of the Eastside, then could drift into portions of the Westside,
including the rogue, Illinois, Applegate and eastern Douglas County
towards Sunday evening. This is because the 700mb flow will be
easterly and 700-500mb flow southeast. So we'll keep a slight chance
for these areas, but confidence on exact location of storms in these
areas remain low. Meanwhile, the models show greater instability
along the northern Cascades Sunday afternoon and expanding south
Sunday evening, so the chance for thunderstorms are higher.
Confidence is not super high that nothing will happen over the west
side where convective available potential energy exceed 1000 j/kg but inhibition should also remain
somewhat high. Storms may be quite dry over the east while anything
to develop across the west will have much more moisture to work
with. Activity tonight should diminish toward midnight but there's a
good chance for isolated storms to continue overnight mainly along
the Cascades and portions of the Eastside. Of note, Special Weather Statement lightning
progs show a higher probability of 10 strikes stretching west into
the Rogue Valley. This looks reasonable given the models are showing
some weak shortwaves moving north to northwest around the upper low.
Therefore have expanded the slight change coverage to include the
Rogue Valley. Elsewhere, we have a slight chance of showers.

The models are in general agreement Monday and Tuesday will be the
most active days in terms of thunderstorm coverage and chances. The
upper low will remain just off the California coast, but it will tap
into more moisture on Monday. Instability is still greatest along
and east of the Cascades, but there is some evidence portions of the
Westside, including the rogue and Illinois valleys could see more
frequent storms. Also there is a better chance for wetting rains as
precipitable waters are expected to increase with a swath of 1+ inches in these
areas Monday afternoon. Also winds at 700mb will be lighter,
therefore storms could be slow movers.

Nocturnal storms again are possible along with lingering overnight
showers and some remaining cloud cover Monday night as the upper low
slowly moves onshore and multiple shortwaves rotate ahead of it.
Right now it's unclear how the area of storms shapes up, but we
could see a fair amount of activity.

Tuesday will be another active day with showers and thunderstorms
over most of the area as the upper low slowly drifts northeast into
northern California. 700 mb winds are expected to be on the light
side (between 5-10 kts) so we'll once again be dealing with slow
moving storms. Moisture is plentiful, therefore the potential for
wetting storms are high with locally heavy rain possible where
storms are at.

The threat for nocturnal storms exist again Tuesday night as the
upper low moves over our area. Right now, we think the focus for
storms will be over most locations Tuesday evening, then shifting
east into lake and eastern Klamath and Modoc counties late Tuesday
night, but details on this could change, so watch for updates.

We do have some lingering thunder chances into Wednesday with
wavering confidence on the low actually moving fully out by then.
However the latest GFS ensembles mostly support a dry day with storm
activity likely off to the east.

Dry weather will return Thursday and could last through next
weekend. The models have come into agreement showing the ridge
building westward into our area in this time period with
temperatures heating back up Friday into next weekend with triple
digit readings in some inland Westside locations and lower to mid
90s east of the Cascades. -Petrucelli


Aviation...23/18z taf cycle...along the coast and over the coastal
waters. Areas of IFR cigs will burn offshore today but persist over
the coastal waters. The lower conditions will return to the coast
this evening and persist into Monday morning. Over the remainder of
the area...VFR conditions will persist...but isolated to scattered
thunderstorms will develop this afternoon. The activity will
diminish tonight but not end completely...continuing on into Monday.
Gusty winds can be expected with the thunderstorms and there may be
some obscuration of higher terrain as well. The Cascades/Siskiyous
and other mountains east of the Cascades will be the favored area
this afternoon and evening. -Jrs


Marine...updated 200 PM PDT Sunday 23 July 2017...a thermal
trough will remain near the coast along with offshore high pressure.
This combination will produce gusty north winds and steep seas, with
gales continuing south of Cape Blanco through Monday night. Very
steep and hazardous seas will develop south of Reedsport by this
afternoon, expanding to nearly all areas by late tonight. The
thermal trough will persist into early next week, though it will
weaken Tuesday and Wednesday. Winds will diminish slightly, but seas
will remain elevated as fresh swell remains in the area. Hazardous
seas and small craft conditions will likely remain in the area
through at least Wednesday. /Br-y


Fire weather...updated 100 PM Sunday 23 July 2017...the main
concern over the next several days continues to be thunderstorms.
The current red flag warning will be expanded into more of zone 625
this afternoon. The watch currently in effect for Monday afternoon
and evening will also be expanded in area as well as extended
through 18z Tuesday due to forecast nocturnal thunderstorm activity.
Finally, a watch as been issued for Tuesday afternoon and evening.

The probability and coverage of thunderstorms will peak on Monday
evening and Tuesday evening. The increase in instability will be
accompanied by an increase in moisture and rainfall with the storms
could be near wetting rain.

Thunderstorm potential covers a much larger area than described in
the fire weather warning/watch. The area reflects forecasted lals of
3 or more. The thunderstorm threat will diminish Wednesday afternoon
and evening as the upper low moves out to the east of the area and
westerly flow develops over the area. This however will likely lead
to breezy west winds during the second half on next week. -Jrs


Mfr watches/warnings/advisories... flag warning until 11 PM PDT this evening for
Red flag warning from 11 am Monday to 11 am PDT Tuesday for
Fire Weather Watch from Tuesday morning through Tuesday evening
for orz617>625. flag warning until 11 PM PDT this evening for caz285.
Red flag warning from 11 am Monday to 11 am PDT Tuesday for
Fire Weather Watch from Tuesday morning through Tuesday evening
for caz280-281-284-285.

Pacific coastal waters...Small Craft Advisory until 11 PM PDT Monday for pzz350-356-370.

National Weather Service Glossary of Abbreviations