Scientific Forecaster Discussion

Return to Local Conditions & Forecast

Without Abbreviations
With Abbreviations

Area forecast discussion 
National Weather Service Medford or
927 am PST Sat Jan 31 2015

Discussion...the latest visible image shows low clouds and fog in
the usual places this morning. Fog will be dense in the Rogue
Valley from Medford to grans pass and into the Illinois Valley
this morning. The fog should be less dense late this morning, but
locally dense fog is still possible. Therefore leave yourself
plenty of room ahead of you while driving.

Elsewhere skies are clear with a few high clouds moving in from
the west over the marine waters. The fog layer is shallow this
morning and the morning sounding also indicates a shallow moist
layer with warmer temperatures aloft compared to yesterday. Low
clouds and fog should burn off around noon in the Rogue Valley,
but is likely to linger for most of the afternoon around Grants
Pass...Illinois Valley and Umpqua basin. There's even a chance it
may not break out at all in grans pass and Roseburg. For now no
changes will be made to the forecast this morning, but we'll
monitor and adjust the forecast if there's evidence the clouds
will burn off in those areas.

Our next focus will be the next front expected to move into the
area Sunday morning. This front is weak and should bring generally
light quantitative precipitation forecast. The exception will be over the northern Cascades, but
even then were not expecting anything significant. -Petrucelli


Aviation...IFR and LIFR ceilings are present in the Illinois...rogue
and Umpqua basin. Unlike yesterday the cloud layer is shallower
and it's clear in Ashland. Therefore there is a chance for the
low clouds and fog to break up between 20-21z at Medford which is
in line with current guidance. However the low clouds are expected
persist for most, if not the rest of the day in Grants Pass and
Roseburg. IFR/LIFR ceilings return this evening and continue into
Sunday morning for the same areas, but there's a chance it could
be limited if there's sufficient middle level cloud cover ahead of
the next front. Elsewhere VFR conditions will continue for the
next 24 hours. -Petrucelli


Marine...updated 630 am PST Saturday, January 31, 2015... the high
pressure offshore and the thermal trough along the coast
will weaken today. A front will approach the waters tonight and
move across the waters on Sunday bringing light south
winds...with moderate west swell building late Sunday through
Monday. Another front is expected on Monday with increased south
winds reaching Small Craft Advisory levels. Then late in the week a
stronger front may bring gales and very steep seas Thursday and


Previous discussion... /issued 339 am PST Sat Jan 31 2015/

Discussion...this morning features continued high pressure with
morning valley fog and freezing fog...and generally clear skies
outside of the fog. The inversion is a little more compressed this
morning...meaning the fog layer is thinner and should clear off
earlier than yesterday in most of the Rogue Valley but this
shallower inversion in is yielding a thicker...dense fog in the
Illinois...rogue...and likely Applegate valleys. A dense fog
advisory has been issued at npwmfr for this morning. Travelers should use
caution this morning and allow for extra time in foggy
areas...especially in the Illinois and Rogue Valley around Medford where
temperatures are below freezing and some Road surfaces could
become slippery in the freezing fog. Not surprisingly...the air
stagnation advisory at npwmfr continues for one more day in most
west and east side valleys. Grants Pass and the Umpqua basin will
continue to be the places where low clouds clear for just a short
time this afternoon...or not at all...before returning tonight.
Warm conditions are expected at the coast and in the mountains.
Brookings is forecast to reach near 70 degrees today. Ridgetop
temperatures this morning in the Coast Range are in the upper
40s...while under the inversion temperatures are below or near

Sunday won't feature much in the way of significant weather but
represents a big change in the weather pattern of the past few
weeks. The high-amplitude ridging that has been dominating the
region will begin to buckle and allow a weak front to move across
the forecast area. This is just the first rain chance of many
through the next week as the ridge axis shifts eastward and opens
the door to several frontal systems.

On Monday a frontal boundary will drape itself over southern Oregon
or extreme northern California and bring a good chance of rain to
most of the County Warning Area. How far south this boundary sags will mainly
dictate where the focus of heaviest precipitation will be...and how
low snow levels get. The gfs40 suggests the lowest snow levels while
the ec has warmer temperatures. The latest forecast relfects a
blend...with snow levels about 7000 to 7500 feet Monday into
Tuesday. The ec and GFS are in good agreement with ridging and drier
conditions on Wednesday before a windy and wet frontal system takes
aim at the West Coast Thursday into Friday and perhaps more bouts of
heavy rain through the weekend. Thursday is still far enough out
where confidence is low for a high-impact event...but gefs ivt tools
are suggesting the potential for a once in 10-year surge of deep
moisture impacting the coast on Friday. Trends are showing
increasing likelihood for an atmospheric river event late in the
week and into the weekend. In summary...confidence is high for
upcoming wet weather...but confidence is low on exact timing and
intensity of this event.


Mfr watches/warnings/advisories...
or...air stagnation advisory until 1 PM PST Sunday for
Air stagnation advisory until 1 PM PST Sunday for
Dense fog advisory until 10 am PST this morning for


Pacific coastal waters...Small Craft Advisory for hazardous seas until 10 am PST this
morning for pzz376.


National Weather Service Glossary of Abbreviations