Scientific Forecaster Discussion

Return to Local Conditions & Forecast

Without Abbreviations
With Abbreviations

Area forecast discussion 
National Weather Service Medford or
801 PM PDT Monday Sep 22 2014

Discussion...23/00z NAM in. Lot of mostly middle and high level
clouds over the Medford County Warning Area this evening...with lower clouds along
the coast. Some isolated thunderstorms did develop over Lake
County this afternoon...but they have moved off to the east.

Those thunderstorms were supported by a combination of surface
heating and an upper level short wave. Southwest flow aloft will
continue to increase ahead of a strong long wave trough offshore.
The first front associated with this low will move onshore later
tonight...but it will dissipate as it does so. The coast and Coast
Range will get some light rain out of this but that's about it.
The greater impact from this system will be winds over the east
side and Shasta Valley. A red flag warning is in effect for that.

The storm will really ramp up beginning Tuesday night. The day
shift did a great job summarizing the event and their discussion
follows.

From the Monday afternoon afd...the upper low that will impact US
with moderate to heavy rain and gusty winds is starting to take
shape near 44n/145w. This is going to be a significant event for
the following reasons. First it's rare that a storm of this
magnitude impacting our area this time of the year. Second, were
looking at over 4 Standard deviations above the Standard anomaly
for precipitable waters Tuesday night into Wednesday. Eventually this low will
get cutoff from the main flow, but it's going to tap into
subtropical moisture which will set The Table for moderate to
heavy precipitation. The models remain in pretty good agreement
that the front will be a heavy rain producer with precipitable waters between
1.5 and 1.75 inches impacting the coast Tuesday night into
Wednesday morning. Also the orientation of the upper flow and
front is one that will have enhanced orographic effects which will
wring out additional moisture (like squeezing a sponge) which
could produce 2-3 times the amount of rain over the South Coast
range and mountains as what the models show for quantitative precipitation forecast and precipitable waters . The
highest quantitative precipitation forecast amounts will be in the typical southwest facing slopes
of the coastal mountains where 2-4 inches could fall.

The main question on Wednesday will be the progression of the front
as it moves inland. The NAM is slower than the GFS and ec and given
the upper flow is nearly parallel to the front...suspect the front
will be slower to move. Therefore have leaned towards the slower
NAM solution with bulk of the precipitation along and west of the
Cascades. There is a little more uncertainty with rainfall amounts
inland. The main reason is that we'll have a southerly flow which
will have a downsloping effect which could typically bring less
rainfall amounts to the Rogue Valley and to a lesser extent the
Umpqua basin. The GFS and ec are global based and will not normally
pick up on these details. Therefore have kept the highest quantitative precipitation forecast
values along the mountains and western Siskiyou County. With that
said...the valleys will still get a decent amount of rain with
totals between 0.30-0.60 of an inch.

Keep in mind the timing and progression of the front is important
because of the potential impact it could have on the burn scars from
this years and last years fires. If the front is slower to move
through and moderate to heavy precipitation persist for a longer
period of time..we could be looking at the potential for mud
slides...rock slides and flooding.

The front will slowly March east Wednesday night into Thursday and
the Cascades and east side will get the bulk of the precipitation.
However much will depend on the timing of the front and how much it
holds up as it moves east. Right now confidence is lower on the
timing.

Winds are going to be the next impact. They are expected to be
strongest along the coast...Shasta Valley and along and east of
the Cascades. The models show 700mb winds ahead of the upper low
increasing between 30-40 kts Tuesday morning and it's a good bet
some of this will mix down near the surface in the Shasta
Valley...so would not be surprised if we see peak gust up to 40
miles per hour near Weed Tuesday afternoon into Wednesday. Also gusty winds
are likely along Highway 31 near Summer Lake. Not enough to
warrant a watch or warning...but given that it's been pretty
quiet as of late...the perception is that it will be significant.

If all GOES according to plan, the front will push east of the
Cascades Thursday and weaken. Meanwhile a large upper trough will
work its way into the area with multiple shortwaves moving
through and cooler air aloft moving overhead. This will result in
scattered showers and below normal temperatures. Winds are not
expected to be as much of a concern.

After Thursday...the models begin to diverge. The GFS continues
to progress the upper trough faster than the ec and shows a ridge
axis building near 140w Friday...shifting towards 130w Saturday.
In contrast...the ec is slower with the progression of the upper
trough and keeps it over the area Friday into Saturday morning,
then pushing east Saturday afternoon with weak ridging Saturday
night. Any ridging is short lived as another upper trough swings
north of the area Sunday. Due to the low confidence in the
extended...made little or no changes beyond Thursday. -Petrucelli

&&

Aviation...based on the 23/00z taf cycle...most inland areas will
remain VFR through Tuesday morning. Smoke will also cause locally
reduced visibility around existing wildfires.

Areas of MVFR ceilings will persist along the coast, and will push
inland overnight as a weak front approaches and moves
onshore. This front will bring some light rain and patchy fog
into Tuesday morning. Little, if any precipitation will make
it inland. A more significant front will arrive at the coast Tuesday
evening with gusty winds developing and the potential for heavy
rainfall and significant visible/ceiling reductions. Rain will spread inland
Wednesday. -Bpn/spilde

&&

Marine...updated 230 PM PDT Monday 22 Sep 2014...southerly winds will
increase later today ahead of a cold front, which will bring periods
of rain, strong winds, and heavy seas to the coastal waters. Winds
will continue to increase through tomorrow, reaching gale strength
over much of the area by the afternoon. Gales are expected to end
overnight, but winds will remain gusty through Wednesday morning,
and small craft advisories for winds will remain through that time.
Seas will gradually build through tonight, with very choppy and
steep seas expected by tomorrow afternoon. Have issued a hazardous
seas warning for all but the far southern and far inner waters into
Wednesday morning. At that time, wind driven seas will diminish, but
a heavy westerly swell will build into the area, keeping Small Craft
Advisory conditions continuing through at least Thursday. Unsettled
weather will continue through the remainder of the week.

&&

Mfr watches/warnings/advisories...
or...red flag warning from 11 am Tuesday to 11 PM PDT Wednesday for
orz624-625.

California...red flag warning from 11 am Tuesday to 11 PM PDT Wednesday for
caz285.
Red flag warning from 11 am to 11 PM PDT Tuesday for caz281-284.

Pacific coastal waters...Small Craft Advisory for winds from 5 am
Tuesday to 11 am PDT Wednesday for pzz350-356-370-376.
Hazardous seas warning from 2 PM Tuesday to 8 am PDT Wednesday
for pzz350-356-370-376.
Gale Warning from 2 PM Tuesday to 2 am PDT Wednesday for
pzz350-356-370-376.
Small Craft Advisory for hazardous seas from 11 PM this evening
to 11 PM PDT Thursday for pzz350-356-370-376.

$$

15/15/15

National Weather Service Glossary of Abbreviations