Scientific Forecaster Discussion
Area forecast discussion
National Weather Service Elko Nevada
313 am PST Friday Dec 13 2013
Synopsis...a weak weather disturbance will push across northern
Nevada today into Saturday bringing a slight chance of snow
showers to the northern third of Nevada. Temperatures will rise a
little through the weekend. High pressure rebuilds into the area
on Friday afternoon and will remain over Nevada for dry conditions
through next Tuesday.
Short term...today through Sunday. Models in good agreement
during the short term but that is not necessarily Good. Water
vapor shows short wave axis extending from around sf Bay area
north-northeast western Oregon...and into the British Columbia
rockies. Seems also to be rotating into a north-south orientation
which...if this continues...could spell a negative tilt by later
tonight or tomorrow. It's moving fast so will not likely affect
Nevada but there you go. The models do not show this at all. At
12z this morning...ec/GFS/nam12 all show much weaker short wave
oriented from central cali coast...through northern Nevada...to
northern rockies in Montana. Hence...they like a faster morning
short wave with weaker influence. Influence of the short wave is
in advance of the trough axis...and is a thick cloud shield over
central and northern Nevada. There Ara a few very light snow
showers over Humboldt and northern Elko County moving quickly
east. Biggest effect of the short wave will be to hold temperatures up
this morning...and block out all views of the meteor shower.
This short wave...which appears to have more energy (and only the
very limited moisture as has been forecast) than forecast is
pushing the southwest US low eastward faster than thought. One fly
in *that* ointment is that the models have been forecasting that
low to open into a wave and forging east. But water vapor shows
that low to still be close with a vigorous enough circulation to
produce overnight thunderstorms...even with positive
strikes...over northern Arizona. Arizona low and the pacnw trough is
producing weak frontogenesis over Nevada...but the pressure
gradient over the state is weak so that drainage winds prevail.
However...as is often the case with marine frontal systems...the
dewpoint gradient easily id's the broad surface trough associated
with the front. There is a tight Theta-E gradient between the
cooler air to the southeast and marine air to the northeast.
All this is contributing to the forecast problem o' the day: what
high temperatures for forecast? Some minor temperatures busts the last couple
days. So have gone with the last couple days' temperature progression
from min to maximum...along with the infrared satellite views of the back
edge of the thickest cloud band already over northwest Nevada.
Speed tool and visual checks indicate that the back edge of the
lower clouds is eroding as it moves southeast. Hence...have
allowed for eventual partial sunshine moving across the state and
have raised high temperatures today and tomorrow a few degrees...Sunday a
couple degrees. Kept low temperatures largely unchanged. Bb
Long term...Sunday night through Thursday. Most of the long term
is quiet with a ridge in control...but things turn much more
active at the end of the period. On Sunday night a ridge axis is
aligned northwest to southeast along the California coast. This
places our area under northwesterly flow aloft. A closed low west
of the ridge axis will gradually move eastward during the long
term...though the GFS and European model (ecmwf) are not in good agreement about
when or where this feature actually moves ashore. The 00z GFS is
about 24 hours slower and several hundred miles further south with
its arrival on the coast as compared to the European model (ecmwf).
However...that's not the main disruption to the weather pattern.
Instead...its the large trough digging southward along the Pacific
coast later Wednesday and Thursday which will bring a significant
weather change. As with the closed low...the GFS is slower
bringing this trough into the region as compared to the European model (ecmwf).
However...that both models have the strong trough and have had it
for several runs is significant. Therefore...after several days of
mostly sunny and dry weather to start next week...cloudier and
snowier weather becomes more likely Thursday.
The temperature forecast is highly problematic. With the ridge in
the area for the first portion of the week...highs may be chilly
as an inversion may persist in the valleys...so our forecast may
be significantly too warm. Kept highs below guidance values
because of this but quite possibly not nearly far enough below.
Lows may suffer a similar fate. A lot of this will come down to
how much snow cover is left by early next week...which is still
not certain. After the deep trough moves in
Thursday...temperatures will nose-dive again. Rcm
Aviation...a weak front passing across the region will result in
clouds today but not much else. VFR ceiling and visible at all terminals
through the next 24 hours. Light winds except at ktph where Post
frontal gusty northwest flow will develop. Rcm