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Threats Assessment Discussion:
Valid Friday, March 31, 2000 - Friday, April 7, 2000.

This product is intended to provide emergency managers, planners, forecasters and the public advance notice of potential threats related to climate, weather and hydrological events. It integrates existing NWS official medium- (3-5) day, extended- (6-10 day) and long- (monthly and seasonal) range forecasts, and hydrological analyses and forecasts, which use state-of-the-art science and technology in their formulation.


The moderate cold event in the Tropical Pacific continues. All available long-range forecast models, both statistical and dynamical, predict continuation of a significant cold event into early 2000. The most recent available (dynamical) coupled ocean-atmosphere model strengthens SST anomalies during the winter and then weakens them by spring. Statistical models also indicate that La Nina conditions strong enough to impact the climate are likely to continue into spring 2000, with rapid weakening thereafter. The forecast for November-December-Janurary 1999-2000 is strongly influenced by the expected average anomalies associated with La Nina, based upon the historical record.

Recent global circulation highlights include:

La Nina continues.

The Sahel region of Africa continues to receive precipitation, but except for the extreme southeastern part, amounts are diminishing as the rainy season draws to a close.

Conditions for tropical storm development in the Atlantic are becoming less favorable, as the westerlies are beginning to move south. SSTs are still high enough to support tropical activity at low latitudes where the vertical wind shear is still weak.

Five named storms developed in August, three formed during September, and two more have developed so far in October, for a total of ten so far this season. The bulk of the Atlantic tropical storm and hurricane activity is typically observed between mid-August and mid-October, so the worst of the season is probably behind us. However, according to the extended range Tropical Storm and Hurricane Outlook issued by NOAA's Climate Prediction Center, Hurricane Research Division and National Hurricane Center, there is a strong likelihood of above-normal tropical storm activity over the Atlantic basin throughout the tropical storm season. Late-season storms, although usually less frequent and less severe than those during the height of the season, can occasionally be very severe and damaging, as Hurricane Mitch last year.

Click here for a summary of some average La Nina properties during Northern Hemisphere winter.

200 mb height anomalies

200 mb winds and anomalies

850 mb winds and anomalies

Outgoing Long wave Radiation (OLR), 5-day mean (Blues imply deep clouds, browns-reds imply few clouds)

The upper-level (200 mb) observed height anomaly map for the past month indicates a fairly narrow zone of negative anomalies over the far northern Pacific over near the Bering Sea and Alaska, and positive anomalies over practically the entire span of the mid-latitude Pacific except for a break where an anomalous trough has recently formed north of Hawaii. Weak below normal heights are shown over low latitudes of both hemispheres. The 200 mb winds for this same period show a seasonally strengthening East Asian jet continuing eastward across the northern Pacific basin to the Alaskan panhandle. The wind anomalies map shows anomalously strong anticyclonic flow aloft over mid-latiudes of both the western and eastern Pacific, with stronger than normal flow diving southeastward into the western U.S. from the later area. The low-level (850 mb) winds for the previous month have been slightly above normal over most of the northern Pacific, with a notable cyclonic circulation over the Gulf of Alaska, reflecting the frequent intense storms that have affected that area.

Observations of 200 mb winds (contours) and 700 mb storm activity (shading) indicate that storm activity has been embedded within the active northern Pacific jetstream throughout the previous two weeks, as well as over the northeastern U.S. and Canadian Maritimes, expecially within the past week. A new area of increase storminess has developed over the central Pacific at relatively low latitudes. The forecast map for days 1-7 storm activity and predicted mean wind speed has been fixed and should be now be available (1230 EDT 10/20/99). It shows a progression and southward shift of the jet over the western and central Pacific and a highly amplified anticyclonicly-curved jet over the western Atlantic, with active storminess continuing at middle latitudes.

Shading on this latter figure is the MAXIMUM of the absolute value of the difference between the v- (north-south) component of the wind, and the 7-day average, V, of v, i.e., |v'| = max|(v - V)|, at 00 and 12 UTC for the seven-day period indicated to the right of the figure when |v'| was at least 12 m/s. The boundary between no shading and the lightest color marks the first shaded contour, which represents 12 m/s, the second contour is 16 m/s, the third is 20 m/s, etc...

2. 6-10 DAY Outlook (Mon April 3 through Fri April 7, 2000).

From Monday to Friday, todays MRF model run suggests a deamplification of the strong mean western ridge and eastern trough pattern. After several intense storms from the Pacific are expected to affect southern Alaska beginning this weekend and continuing well into next week, the associated upper troughs are expected to begin to break down the western ridge, perhaps bringing the first significant precipitation in several weeks to the Pacific Northwest. High onshore winds, heavy precipitation, and high waves along the coast will accompany these storm systems. A second shot of abnormally cold air will probably affect the northeastern part of the Nation in the latter part of the week.

Click here for a discussion of the MRF Ensemble forecasts.

3. 3-5 DAYS (Fri March 31 through Sun April 2, 2000).

From Friday to Sunday, Hurricane Jose is unlikely to affect the U.S. mainland as it will probably recurve well out over the Atlantic. Check the latest advisories issued by the National Hurricane Center in Miami. The strongest cold air outbreak of the season so far is predicted to move southeastward from Canada into the Great Plains and then overspread much of the eastern part of the Nation, bringing the first killing frost or freeze of the season to many areas where the growing season has not yet ended, including interior sections of the southeastern states. If the trough aloft is as strong as predicted by most of the models, this airmass may be cold and unstable enough to bring the first Lake Effect snows of the season to some locations, although the lakes are probably too warm for significant accumulations. No significant precipitation will fall over the drought area in the interior Pacific Northwest or over areas of short-term dryness in California and Nevada. Above-normal temperatures in both states will aggravate fire danger. Drought will continue in Texas after slight relief over the past weekend from cooler weather and some precipitation. Below normal temperatures in remaining drought-affected regions in the Tennessee and Ohio Valleys and the Southeast may somewhat mitigate effects of the dryness in those areas.