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Hazardous Weather Ahead and
Outlook for Winter 2009-2010
Dave Reynolds -MTRDan Keeton - STOMeteorologists in
Charge
weather.gov/SanFrancisco
or /Sacramento
Your NWS Weather Forecast Office
San Francisco Lightning
• Our goal:
Provide forecasts and warnings so people can make informed decisions.
WFO Product Timeline
7-Day Forecast
3-5 Day Outlook
Special Weather StatementHazardous Weather OutlookMarine Weather Statement
Hydrologic Outlook
Hazard Watch12 to 48 hours ahead
Hazard Advisory Hazard Warning
potential
imminent
Nuisance Life and/or property threatening
• This weather pattern, which features strong zonal flow aimed at portions of the west coast, is likely to last up to a week and maybe longer.
• The pattern may be related to ongoing activity in the tropics associated with the Madden-Julian Oscillation.
• This weather pattern is likely to be more transient in nature than that typically observed with El Nino.
• El Nino does favor additional events of this type – but typically they occur later in the winter (January-February timeframe), and are associated with an amplified wave pattern, eastward extension of the Pacific Jetstream, and persistent pattern of storminess somewhat farther to the south.
Developing Stormy Pattern for California
MJO Index -- Recent Evolution
The MJO has continued to propagate eastward.
The axes (RMM1 and RMM2) represent daily values of the principal components from the two leading modes
The triangular areas indicate the location of the enhanced phase of the MJO
Counter-clockwise motion is indicative of eastward propagation. Large dot most recent observation.
Distance from the origin is proportional to MJO strength
Line colors distinguish different months
SSMI shows AR stretching across Pacific to Central California
Central CA>15 inches rain
Atmospheric River
Series of Storms Starting Sunday Night
First Storm: Sunday Night – Monday NightMore Storms: Tuesday – Next Week Week
Remnants of Typhoon NIDA
MJO driven thunderstorms
Enhanced Jet
Cold arctic air to move south and west Sunday
H
In general there is widespreadagreement on strong zonal flow into California next week. MaybeCentered south of Bay Area.
Valid 4pm Wednesday Dec 09
Winter Outlook
• Expect possible moderate strength El Nino to persist through spring.
• Could bring recurrence to weather we will see next week.
• May not see influence until January• Impacts still not certain
SST Outlook: NCEP CFS Forecast Issued 8 November 2009
The CFS ensemble mean (heavy blue line) predicts El Niño will last at least through Northern Hemisphere spring 2010.
El Niño
El Niños feature a strong jet stream and storm track across the southern part of the United States, and less storminess and milder-than-average conditions across the North.
ENSO strength vs SFD WY rainfall and sign of PDO Sept-Nov.
Strong El Nino
Weak/ModEl Nino
NeutralWeak/ModLa Nina
Strong La Nina
Where are we now? Years with closest fit
DECJAN
JANFEB
FEBMAR
MARAPR
APRMAY
MAYJU
N
JUNJU
L
JULA
UG
AUGSEP
SEPOCT
OCTNOV
NOVDEC
0
10
20
30
40
50
60
70
MEI Ranking
1972-73
2009-10
1965-66
1957-58
Monthly Pairs
Ran
kin
g 1
-60
72-73 WY = +.5965-66 WY = -.2557-58 WY = +.67
Winter Predictionsfor 2009-2010
So what does this forecast mean regarding our current drought situation?
Most of California is currently under drought conditions
Current drought conditions are likely to improve over the next several months
El Niño Generalities
• During El Niño rainfall and thunderstorm activity diminishes over the western equatorial Pacific, and increases over the eastern half of the tropical Pacific.– This overall pattern of rainfall departures spans nearly one-half the
distance around the globe, and is responsible for many of the global weather impacts caused by El Niño.
• El Niño episodes feature large-scale changes in the atmospheric winds across the tropical Pacific, including reduced easterly (east-to-west) winds across the eastern Pacific in the lower atmosphere, and reduced westerly (west-to-east) winds over the eastern tropical Pacific in the upper atmosphere near the tropopause.
El Niño’s affect on the Atmosphere• An eastward extension and equatorward shift of the jet stream from
the International Date Line to the southwestern U.S.
• A more west-to-east flow of jet stream winds across the U.S.
• A southward shift of the storm track to the southern part of the United States.
• A southward and eastward shift of the main region of cyclone formation to just west of California.
• Results: a better chance of a stormy winter and increased precipitation across California and the southern U.S, and less stormy conditions across the northern part of the U.S.
• Also, there is an enhanced flow of marine air into western North America, along with a reduced northerly flow of cold air from Canada to the U.S. These conditions result in a milder than normal winter across the northern states and western Canada.
El Niño: pronounced eastward extension of deep tropical convection and heating to well east of the date line which extends the subtropical ridges in both hemispheres to well east of the date line. This results in a pronounced eastward extension of the midlatitude jet stream to the extreme eastern Pacific and an equatorward shift of the jet streams over the eastern Pacific. This in turn has a major affect on the winter weather patterns and storm tracks.