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Area forecast discussion 
National Weather Service Medford or
804 PM PDT Monday Apr 20 2015

Update...the last of this string of warm days has come to an end,
with cooler temperatures and a chance of rain and/or thunderstorms
on tap for tomorrow. For this evening and tonight, the inheritedforecast
is right on track, with only a few adjustments needed to bring the
cloud forecast in line with the current observations along the
coast. For more information on the rest of the forecast, see the
previous discussion below. -Bpn


Aviation...for the 21/00z taf cycle...along the coast and in the
Umpqua basin...IFR ceilings along the coast will persist and spread
inland to the Umpqua basin late tonight through middle-morning
Tuesday with higher terrain becoming obscured. Skies will clear to
VFR by Tuesday afternoon. Elsewhere...VFR conditions will prevail
through Tuesday...but isolated thunderstorms will develop Tuesday
afternoon from the Cascades east and Siskiyous south. -Jrs


Marine...updated 800 PM PDT Monday Apr 20 2015...north winds and
choppy seas have eased somewhat this evening, but will persist
through the overnight hours for the waters south of Cape Blanco.
Meanwhile, near shore fog will thicken again tonight for almost
the entire southern Oregon coastline. Winds and seas will increase
Tuesday afternoon ahead of a cold front, which will move onshore
Tuesday evening. Seas will reach a peak Wednesday and remain
hazardous through Wednesday evening. Conditions will briefly
improve Thursday as offshore high pressure and the thermal trough
return, then a series of weak systems will move onshore Thursday
night into Saturday. -Bpn/jrs


Previous discussion... /issued 209 PM PDT Monday Apr 20 2015/

Discussion...skies remain clear over most of the area into this
evening, except for marine stratus along the coast and marine
waters. It's possible the stratus will Peel back away from the coast
for a few hours this evening only to return tonight. Also we could
see some cumulus build ups over the higher terrain in southern
Siskiyou County late this afternoon and evening.

The ridge and thermal trough shifts east on Tuesday as an upper
trough near 140w will move east. At the surface a cold front will
reach the coast Tuesday morning, then move inland during the day.
The front is not expected to be a big rain producer, but it will
bring significant cooling (at least relative to what we experienced
the last several days) west of the Cascades. The models continue to
show sufficient instability for a chance of thunderstorms along and
east of the Cascades and northern cal Tuesday afternoon into Tuesday
evening. It's in these areas where there is a better chance for the
cap to break. Also the NAM and GFS show a shortwave passing by lake
and Klamath County late Tuesday afternoon and evening and shows a
bulls eye area of higher quantitative precipitation forecast and middle level lapse rates near 8
c/kg, thus raising the possibly that some of these storms could be
moderate. Meanwhile the cap should be stronger and this combined
with the influx of marine air, should be sufficient to prevent any
thunderstorm development west of the Cascades.

The greatest instability shifts east of the area Tuesday night and
showers will be on the decrease with most confined to the Eastside.
The upper trough will progress east over the area and at the same
time split. The models have trended drier on Wednesday and thus have
reduced pops, but still kept in a slight chance for east of the
Cascades. Also it's not looking as unstable, so have taken out the
mention of thunder.

Weak ridging builds southwest of the area Wednesday night, but that
will be short lived with a westerly flow and weak troughing
returning on Thursday. Another weak front will bring some showers
for the north coast Thursday afternoon, then the front will move
inland Thursday night bringing a better chance for showers mainly
north of the Umpqua Divide. -Petrucelli

Long term discussion...Friday through Monday...details about the
extended portion of the forecast are becoming clearer as models have
exhibited clear trends over the past several days. First...there is
increasing confidence that Friday will feature an upper trough axis
moving through from west to east...bringing a chance of showers for
most of the forecast area...though rain and mountain snow amounts
should be light. Models suggest some enhanced precipitation possible
in northeast California and Southern Lake and Klamath counties...and
given good agreement in this idea...have bumped up probability of precipitation there.

Friday night into Saturday morning should feature shortwave ridging
and dry conditions behind the trough...followed by warm frontal
precipitation that looks more and more like it will affect the
northern half of the County Warning Area and leave the other half dry. Significant
upper troughing that's been hanging over the northeast Pacific
finally reaches the forecast area with significant rain potentially
Monday into Tuesday. That said...there is high uncertainty as to
when this troughing reaches the area. Ensemble members are out of
phase...with some keeping the trough axis well out to sea...and thus
delaying any significant precipitation.

In short...the extended forecast has evolved from one that was
suggesting a wet and cool late part of the weekend to one that is
warmer and drier for at least the southern half of the forecast
area. /Sk


Mfr watches/warnings/advisories...


Pacific coastal waters...Small Craft Advisory for winds until 5 am PDT Thursday for
Hazardous seas warning from 2 PM Tuesday to 5 am PDT Thursday
for pzz356-376.
Gale Warning from 2 PM Tuesday to 5 am PDT Thursday for
Small Craft Advisory for hazardous seas until 5 am PDT Thursday
for pzz356-376.
Small Craft Advisory for winds from 2 PM Tuesday to 5 am PDT
Thursday for pzz350-370.
Small Craft Advisory for hazardous seas from 2 PM Tuesday to 5
am PDT Thursday for pzz350-370.



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