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HMT Forecast Discussion for Monday 25 Feb 08 (issued 1740z/Monday)

The storm has come to an end with most of the precip winding down around
04z to 06z but some lingering higher elevation snows finally ending at
KBLU around 12z this morning. Quite a lot of precip yesterday even
though the surface low and upper level system were weakening as the
storm came onshore in far nw CA during the afternoon. Some totals for
the event from the ESRL gages include: Alta, ~2.6" and Big Bend ~4.3";
the BLU precip gage was down for Sunday but the snow gage was working
and it looks like a storm total of near 20" or so. The closest ski area to
KBLU reported almost 4 feet of snow according to WFO SAC.
CNRFC data has storm totals of 2.86" at KBLU (1.05" and
1.81" in the two 24-h periods ending
at 12z/24 Feb and 12z/25 Feb), 0.80" at KSAC (0.24" and 0.56"
respectively), and 2.76" at Huysink (1.32" and 1.44" respectively). It
snowed heavy at times at KBLU on Sunday but around the time of the
trough passage in the evening the temperature at the KBLU METAR warmed
to 34F along with a supposed changeover to some light rain and mixed
precip from 03z to 05z/Mon. This time it could be real given the warmer
temperatures, as opposed to the mixed precip reports from the KBLU METAR
late Saturday night that were pretty much discounted in yesterday's
telcon as being heavier and wetter snow falling. WFO SAC forecasters
were not sure about the changeover this time, since the temps were warmer
it was feasible. Winds were brisk but not excessive yesterday at KBLU,
peak gusts from the S to SSE of 33-34 kts from 21-23z, matching the
strongest winds that occurred earlier in the storm between 03-10z/Sun.
Winds gradually decreased later in the
day with a shift to a 200-210 wind in the evening with the warmup and
trough passage, then things pretty much calmed down. So with the big
occlusion there was not a dramatic frontal passage like we saw with the
more concentrated early January IOP, but certainly a good system with
some interesting complexities, especially in regards to rain vs. snow at
the radar site (a close call at times, and a tough forecast for sure).

Onto the current weather, the Pacific remains active and this will be
the case over the next 2 weeks. Despite this the ARB may get shut out
from much action as the systems weaken as they approach a mean ridge
forecast to be near the West Coast, though with all the activity in the
Pacific there is uncertainty as we go out farther in the forecast.
Right now the weakening system that was a very strong wave in the
mid-Pacific is a north-south trough near 150W with a nice tap to
tropical moisture, but forecast to continue to weaken and split, with
the main portion riding north of CA where a ridge is now in place, and
another piece becoming a very weak cutoff that approaches the West Coast
midweek but with diminishing moisture. West of this system is another
trough near 165E with a nice tap back to the sw all the way to the sw
Pacific. Behind this is another system about to emerge off of Asia.
The system currently near 160E is the one that brings a threat of
something by Sat/1 March, but even the more favorable models, such as
the 00z/ECMWF and 00z/Global Canadian, have this as a fast-moving event
with the heaviest precip to the north of the ARB, so probably not
IOP-worthy, but certainly worth watching this week given its current
nice moisture tap into a tropical source. The GFS ensembles from 00z
have good agreement in keeping this system north of the ARB, but there
are some members of the Canadian ensemble system that do bring some
precip into the ARB, though even these have a fast-moving event.

The next in the line that will come off of eastern Asia is predicted to
wrap up into the Gulf of Alaska later this week with most of the energy
again staying to the north. Timing and strength differences show up
with this one for very early next week but at this time this system also
is forecast by the deterministic runs to be too weak and fast-moving.
Ensembles generally agree, again, a few are close to bringing something
to the ARB. The models generally forecast this wave to then dive into
the Intermountain West and amplify midweek of the first week of March,
putting an amplifying ridge along the West Coast. This solution has
pretty broad support in the 00z ensemble forecasts. However, a lot of
energy is forecast by this time period to be in the mid to western
Pacific, and the latest deterministic 12z/GFS finally pushes some of
this into the West Coast around 10-11 March, as do some members of the
00z ensembles. Right now the ensembles would be pointing to the best
potential for an active ARB for the second week in March, with some
close calls possible in the first week.

ed szoke NOAA/GSD/CIRA