MOUNTAIN WEATHER FORECAST FOR THE OLYMPICS WASHINGTON CASCADES AND MT HOOD AREA NORTHWEST WEATHER AND AVALANCHE CENTER SEATTLE WASHINGTON 230 PM PST MON JAN 06 2009 WEATHER SYNOPSIS FOR TUESDAY AND WEDNESDAY
AND MT HOOD AREA
NORTHWEST WEATHER AND AVALANCHE CENTER SEATTLE WASHINGTON
230 PM PST MON JAN 06 2009
WEATHER SYNOPSIS FOR TUESDAY AND WEDNESDAY
Substantial moisture continues to stream over a very flat offshore ridge early Tuesday afternoon, with a very strong jetstream of 150-170 mph pushing the trailing portion of a strong warm front over the region. This should be akin to opening the main valve on a very large fire plug and should result in increasingly heavy rain or snow in most areas Tuesday afternoon at substantially rising freezing levels and increasingly strong winds.
As the offshore ridge flattens further and moves over the NW coast late Tuesday, the front is expected to stall over the region Tuesday night and much of Wednesday as minor disturbances ripple along its back edge offshore…
Global Scale: High and Low pressure systems develop
Mean sea level pressure for June, July, August, top and December, January, February, bottom.
Location of high or low pressure centers provides information about:
The change in pressure measured across a given distance is called a "pressure gradient".
Winds blow clockwise out of a high pressure center.
Winds blow counterclockwise into a low pressure system
A high pressure center is where the pressure has been measured to be the highest relative to its surroundings.
That means, moving in any direction away from the "High" will result in a decrease in pressure.
A high pressure center is indicated on a weather map by a blue "H".
A low pressure center is where the pressure has been measured to be the lowest relative to its surroundings.
A low pressure center is indicated on a weather map by a red "L" and winds flow counter-clockwise around a low in the northern hemisphere.
As air flows out of a high pressure center, air from higher in the atmosphere sinks to replace it.
This subsidence produces warming, dissipation of clouds and precipitation
As air converges in a low pressure center, it rises and cools. If moist, cooling will cause moisture to condense and form clouds.
[The average atmospheric pressure at sea level is ~1000mb]
At every point along a given isobar, the values of pressure are the same.
An area of relatively high pressure is centered in western S Pacific, while the pressure increases outward from this region.
An area of lower pressure is located in western B.C. & Ak.
Ridges: Elongated high pressure centers.
The small green numbers are contour labels, which identify the value of an isobar.
Small wave length waves or ripples are called short-wave troughs or ridges.
500 mb forecast for 1/21/1999. Troughs = red; solid lines = 500 mb height contours.
station reporting symbol
Observed Dew Point Temperature
The amount of cloud cover at the time the observation. In this case, broken clouds were reported.
Sea Level Pressure:
The last three digits of the sea level pressure reading in millibars (mb).
The wind barb. The wind barb indicates wind direction and wind speed.
Use available surface data
Dew point temperature
Wind direction and
speed (wind barb)