Scientific Forecaster Discussion

NWS Discussion
			
				

Southeast Alaska forecast discussion 
National Weather Service Juneau Alaska 
629 am akdt Thursday may 23 2013 


Short term...this was a tricky forecast defined by relatively 
light flow and poorly defined features in the models and on other 
tools like satellite. Let US start with what we know. We are in a 
warm pattern with lots of southeasterly flow aloft and 
temperatures will once again warm well into the 60s across the 
region today. A few interior locations will likely see the 70s 
like deeper parts of the Mendenhall Valley. Think in the far 
north like Skagway and Haines, temperatures will recover now that 
clouds have moved north. With weakening high pressure over the 
Gulf of Alaska and a thermal trough of low pressure extending 
northwest, most winds will be northwesterly across the Panhandle. 
As interior parts warm rapidly this afternoon, we should see 
another surge of westerlies into the ocean entrances, similar to 
yesterday and thus have bumped up Cross Sound and southern 
Chatham into Small Craft Advisory winds this afternoon, and emphasized some 25 knots 
winds in Sumner without placing them in Small Craft Advisory. Sea 
breezes will develop late morning into the afternoon before 
shutting off in the evening. We also have a lot of sea stratus 
offshore with some fog being reported by mariners. The sea 
stratus reached Yakutat, Sitka, and Cross Sound around midnight. 
This is something we expected and see no reason why the pattern 
will vary much tonight. Fog was added across the offshore zones 
using NAM boundary layer humidity fields and the fact that 
confirmation has been reported. 


Now for the tricky part: sky cover...a lot more high and 
middle-level clouds arrived into the southern Panhandle yesterday 
evening and overnight than previously thought. Cannot say that 
mechanisms weren't revealed: there was some weak vorticity 
advection riding over the ridge aloft overnight coming from the 
southwest. But most of the models did not suggest cloud cover of 
any significance, with the exception of the European model (ecmwf)...which seemed 
to overcompensate for the lack of clouds in the other models. 
While cloud cover on infrared satellite imagery overnight seems to 
suggest weakening...clouds looked thicker than thought on webcams 
and with coordination from Annette wso, elected to call things 
mostly cloudy in the south. But feel as though they will have 
some sunshine this afternoon. The important thing is dry weather 
here. Cloud cover over the central Panhandle seemed thinner and 
less shadow inhibiting, so called for mainly sunny conditions 
here and to the north. For the northeast Gulf Coast, worried that 
middle to high level clouds from the west may tamper with sunshine 
this afternoon as marine layer breaks up...so went with partly 
cloudy for this afternoon for Yakutat. Cloud cover tonight was 
equally challenging. Models...especially GFS showing a surge of 
moisture moving in from the southeast. MOS guidance cloud cover 
has been unhelpful as it has cared little about the current 
conditions over the south. Think some down-sloping will tend 
toward clearer conditions than what models are indicating, and so 
have left things mainly clear for much of the interior Panhandle 
overnight. But partly cloudy skies may have been the safer 
alternative. 


This weak vorticity center over the southeast Gulf is forecast 
to weaken and move north and as it does it will pump more warm 
air over the region from the southeast, and with that inevitably 
comes moisture as well. The moisture will begin to work through 
the southern Coast Mountains this afternoon that may bring the 
threat of an isolated or scattered shower near Hyder. Things 
dissipate quickly over the evening, but a more significant lobe 
of moisture arrives late that will spread scattered showers over 
Hyder toward the early Friday morning hours. For more about this 
onset of some wetter weather for the southern Panhandle see the 
long term segment. 


One final note about temperatures, cooled them a little for this 
afternoon given our thinking at the time that skies would be a 
little cloudier, so it could get warmer than advertised...especially 
over the north. White Pass temperatures were warmed tonight 
thinking an inversion may set in below keeping temperatures at 
pass level relatively mild. Relied on NAM 925 mb temperatures for 
guidance. 


Models were having a difficult time with some of the positioning 
and strength of the surface high offshore, and especially clouds 
over the region. So instead chose not to change pressure field 
and confined wind changes to local effects. Relied on satellite 
interpretation for cloud cover and local knowledge of sea breezes 
for help on winds. Forecast confidence average to good, but falls 
for sky cover...especially over the southern and even the central 
zones. Luckily, impacts of missed forecast will be minimal. 


&& 


Long term...the extended forecast period looks like it will be 
dominated by high pressure aloft centered over western Canada and 
eastern Alaska. The Pacific storm track will stay well south of the 
Panhandle. A cold upper level low center producing rain and mountain snow 
is stuck over the Pacific northwest. Cloudiness is curving around this low 
and moving northwest over British Columbia toward Southeast Alaska. The Panhandle will 
be under this southeast flow aloft through the middle of next week. The 
main forecast challenge will be determining if and how much rain 
will spill over the coastal mountains from British Columbia and 
into Southeast Alaska. This weather pattern favors a bit higher chance of 
rain over the southern Panhandle and drier but cloudy weather over 
the northern Panhandle. There is better agreement in nearly all 
models and National wpc guidance that maintain temperatures will remain 
at or above normal through the extended range. 


The GFS ensemble mean was used to update the grid forecasts. The 
deterministic hemispheric models such as the GFS ec and Gem had 
various solutions so the ensemble mean was is an effective 
compromise for describing the general large scale features. 
Forecast confidence regarding the location and timing of 
precipitation is low. 


The first shot of possible precipitation over Southeast Alaska appears to 
be during the day on Friday over the southern zones. Deformation 
aloft and an increasingly moist air mass could lead to light rain 
especially north and east of Ketchikan. This will likely dissipate and 
dry out as it moves north Friday night. Another batch of rain 
showers could develop Sat and Sat night over the central and 
northern coastal mountains. Related to this I lowered daytime maximum 
temperatures a bit from previous forecasts through the weekend. The chance 
of precipitation does appear to be higher and more widespread by Monday and 
Tuesday but rain amounts are not significant...certainly less than 
the last two very wet weekends in Southeast Alaska. 


&& 


Ajk watches/warnings/advisories... 
public...none. 
Marine...Small Craft Advisory for pkz022-033-041-042. 
Small Craft Advisory due to seas for pkz043. 


&& 


$$ 


Jwa/ta