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HMT Forecast for 4 Jan 2008

Wind and precip are on the increase as scheduled as a very strong jet
and associated wave approach the Northern California coast this morning.
Temperatures have been rising at BLU and are now above freezing as the
heavy precip gets underway, and winds are picking up as well. Last
night's precip was mainly snow but not as much as had been thought
possible. But today will be the big day as the strong jet
approaches...peak winds in satellite obs this morning are a whooping 230
kts near 45N/160W at 34kft, with this strong jet dipping to the
southeast on the backside of the strong upper level system centered near
45N/135W this morning. This is providing enormous diffluence/divergence
aloft that is approaching the ARB and helping to squeeze out the
moisture that had been lurking off the coast. Several extensive ssw-nne
bands of echoes across the northern half of CA and this will come across
the ARB over the next 12 h. At 17z KBLU reporting 37F with S winds
gusting to 46 kts. Strong S winds at lower elevations as well with KSFO
gusting to 57 kts. At 11z some lightning was observed off the coast.
Central pressure in the surface low off the coast of WA is near 960 mb,
with the surface cold front extending to the se to just off the Northern
CA coast (it may have just passed at 17z a buoy to the north of KSFO).
At 1744z KBLU ob was R+, +4C, gust 180/61 kt, generally KBLU thru the
latest 1930z ob has been maintaining R+ with S winds gusting to near 60
kts. The latest obs do show the surface front has progressed inland
some across and north of KSFO with a shift to a more westerly direction
in the winds.

A look at the short-range forecast shows things pretty much on track as
discussed yesterday. Good call by the local folks on the snow level
getting back above KBLU, where it will remain as the really heavy precip
continues this afternoon. The 700 mb flow at ~55 kts at KBLU at 12z is
forecast to peak near 90 kts by 21z with 700 mb temps remaining around
-2C, then the 700 mb front passes near 00z with only a slight shift in
the winds but a wind decrease and temperatures down to -10C by 03z,
according to the latest 12z NAM run. A look at the hourly BUFR
soundings from the 12z NAM shows a sharp drop in temperature in the 800
to 700 mb layer between 00z and 01z. Surface temperatures lower
significantly by 02z, and on the BUFR soundings get to 0C by 06z/Sat.
Given the strong drop aloft it appears precip at KBLU should transistion
to snow between 01-03z. By 12z/Sat the freezing level drops to around
3800 ft, possibly at tad lower at times before a final drop towards 3000
ft on Sunday. Peak precip period is 18-21z with ~1.25" forecast just
west of KBLU (in the NCEP hi-res 5 km runs up to 2" in this period),
close to another inch at KBLU in the next 3h (ending 00z/5 Jan), then a
decrease to 0.3-0.5"/per 3-h period through 12z/Sat, followed by
continued great orographic snows all day Saturday at the rate of about a
third of an inch per 3-h at KBLU, with wsw 700 mb winds 40-50 kts and
temps around -12C. Some predicted precip totals from the models: by
06z/Sat at KBLU NAM has ~3.8", with max of 4.8" to the west, while the
GFS max of ~4" is at the elevation of KBLU (this location difference
could simply be a terrain resolution issue); 06z run of the WRF/NMM ~7"
at KBLU, while the WRF/ARW has ~8". Total precip by the time things
totally taper off ~06z/Mon/7 Jan: ~10" at KBLU in the NAM (max 12.3"
just to the west); ~7.2" max (located near KBLU) in the GFS. The GFS
amounts are more in line with the current HPC forecasts. Per the
conference call the NWS is predicting about 5 ft of snow by early Monday
at KBLU, with up to 10 ft at higher elevations to the West. Discussion
of road conditions (as relates to getting the radar crew out if they
stayed through the event) indicated that folks expected I80 would be
opening up by Monday given the current forecast. Decision though was
ultimately made that the radar crew should leave by ~00z (later today
before dark).

A slight lulll is possible 18z-22z/Sat but then things begin to pick up
again with the approach of "part 3", a shortwave riding se in the flow.
A change from yesterday appears to be that this wave (or perhaps
another right behind it) will not finally clear past the ARB until
~00z/7 Jan (late Sunday), when winds finally shift to the nw at 700 mb
and orographic snows at KBLU decrease to a rate of ~0.1"/3h, with this
continuing through Monday along with a slow rise in 700 mb temps. So
another change in thinking, at least in the NAM, would be a lull in the
precip from ~00z/7 Jan through the next 24 h (so all of Monday daytime).
Overall the above timing looks similar to what is in the 12z/GFS, which
shows a pretty good decrease to the precip after 00z/7 Jan, and
especially by 12z Monday, with fairly dry (or maybe just light snow
showers at KBLU) continuing through 12z/Tue.

The GFS then has the next system picking up by late Tue then pretty much
over by ~15z/Wed, with 1.5-2" indicated at this time for the ARB area.
A minor system may follow by Thu but then ridging still looks to be the
case thereafter through the weekend of 12-13 Jan. The 00z/ECMWF model
is similar to the latest GFS with the current system. For the Tue/Wed
wave it is wetter, and blends it into the next system so that clearing
does not occur until Thu/10 Jan, followed by ridging by the weekend.
The latest Global Canadian model forecast has a break on Monday into
early Tue, then the next system which ends by ~18z/Wed/9 Jan. NOGAPS
12z run is also similar in the timing of these systems, with the end of
the midweek event on midday Wed and nothing further as the next wave
stays farther north than in the ECMWF model run. This midweek system
could be mostly a rain event at KBLU at least as it currently looks,
with the snow level a little above that elevation. The 12z ECMWF is
generally similar to the earlier run and in agreement on the midweek
system, beginning before 12z/Tue and ending by ~18z Wed. The EC run
does have a close call over the next weekend it is shifted slightly
farther west with the ridge so that the next wave brushes across the ARB
instead of being farther east.

Otherwise, there is general consensus in the ensemble forecasts of a dry
period beginning later next week and continuing possibly thru the
following week as an upper level ridge builds along/near the West Coast
and the mean trough shifts east over the CONUS. There is actually
pretty good agreement in most members of the Canadian and GFS ensemble
systems for this longer period. The 12z GFS ensembles paint a similar
picture, basically a dry pattern for the ARB beginning after the midweek
event next week and continuing through the week of 14-20 January.

Bottom line then is a wild time with very heavy rains and strong winds
at KBLU for the next 6-7 h (perhaps 4-5 inches of rain in this period
potentially, then the cold front goes by near 00z with precip changing
to snow and likely blizzard conditions overnight into Saturday. A
possible lull Sat afternoon but then heavier snows late Sat through most
of Sunday, then things calming down with a break on Monday, and some
uncertainty about the significance of the Tue-Wed (or Thu) system.

ed szoke NOAA/CIRA/GSD