Written by Sig Silber
Updated September 2 , 2018 at 9:30 PM EDT with informaition on all the active cyclone tracks related to the U.S.
It is basically a question of how far north tropical activity will impact CONUS weather in September. Both Fall and Tropical Activity patterns are energized. NOAA issues their Early Outlook for the following month on the third Thursday which was August 16, 2018 and then updates it on the last day of the month namely August 31, 2018. So this is our report on the Update for September.
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This is just a reminder that tropical activity in the Pacific is energized right now. It may be due to the onset of El Nino or the warming of sea surface temperatures or both.
Updated with the recent tracks of storms
Tuesday Night or Wednesday
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We will be providing updates of the NHC discussion but a more direct source is here.
Shifting focus to Hawaii
Then the next contender
More aciton in the Atlantic
Here is the link to the general discussion and much other information.
Now let us address the NOAA Update of the September, 2018 Forecast.
NOAA has, as usual, issued an update for the month following the last day of the prior month. This NOAA update was issued today Friday August 31, 2018. In this Update Report, we present the Updated NOAA Outlook for September and compare it to the NOAA “Early Outlook” for September issued on August 16, 2018 more than two weeks ago. Please note: all smaller graphics can be enlarged by clicking on them or right clicking and selecting the “view image) option.
First, I present a summary showing the prior and the new September forecast and the previously issued three-month forecast. Larger graphics follow but these smaller graphics can be enlarged.
Temperature | Precipitation | |
Prior Issued on August 16, 2018 | ||
Updated on August 31, 2018 | ||
Three Month Forecast Issued on August 16, 2018 |
Prior September Temperature Forecast Issued on August 16, 2018
New September Temperature Forecast Issued on August 31, 2018
Turning to Precipitation
Prior September Precipitation Forecast issued on August 16, 2018
New September Precipitation Forecast Issued on August 31, 2018
Here is the NOAA Discussion released on August 31, 2018 with the September Update. The Week 3 – 4 Forecast and Discussion was also updated on August 31, 2018 and that discussion is provided also.
30-DAY OUTLOOK DISCUSSION FOR SEPTEMBER 2018
THE UPDATE TO THE SEPTEMBER TEMPERATURE AND PRECIPITATION OUTLOOKS WAS PRIMARILY ADJUSTED BASED ON SHORT-, MEDIUM- AND EXTENDED-RANGE MODEL GUIDANCE AND TO A LESSER EXTENT SUBSEASONAL FORECAST INFORMATION. THE MJO SIGNAL IS NOT ORGANIZED AND MODEL FORECASTS DUE NOT INDICATE ANY CONSISTENT, RELIABLE REORGANIZATION AND COHERENT EASTWARD PROPAGATION IN THE COMING COUPLE OF WEEKS, SO THE MJO DID NOT PLAY A ROLE IN THE UPDATED OUTLOOKS. OTHER FACTORS THAT CONTRIBUTED TO THE REVISED OUTLOOKS INCLUDED OCEAN SURFACE TEMPERATURES AND DRY SOIL MOISTURE CONDITIONS IN SOME AREAS.
FOR TEMPERATURE, THE ANTICIPATED FORECAST MID-FLOW PATTERN EARLY IN THE MONTH FAVORS TROUGHING ACROSS PORTIONS OF WESTERN NORTH AMERICA THAT SUPPORTS SOME PENETRATION OF BELOW NORMAL TEMPERATURES FOR PORTIONS OF THE PACIFIC NORTHWEST, NORTHERN ROCKIES AND NORTHERN PLAINS EARLY IN THE MONTH. BUT IT APPEARS THAT THIS ACTIVITY WILL BE GENERALLY TRANSIENT IN NATURE AND SUBSEASONAL GUIDANCE FAVORS A RETURN TO MID-FLOW RIDGING AND POSITIVE HEIGHT DEPARTURES IN THESE AREAS. COMBINED TIME SCALE MONTHLY FORECAST TOOLS INDICATE QUITE MODEST PROBABILITIES FOR EITHER ABOVE- OR BELOW-NORMAL TEMPERATURES IN THIS REGION FOR THE MONTH AS A WHOLE SO “EQUAL CHANCES” (EC) IS FORECAST IN THE FORECAST UPDATE.
THERE ARE GENERALLY MINOR DIFFERENCES IN THE ALASKA OUTLOOK RELATED TO THE ABOVE DESCRIBED MID-FLOW SITUATION WITH THE MAIN CHANGE – AN INTRODUCTIONS OF A SMALL AREA OF ENHANCED ODDS FOR BELOW-NORMAL TEMPERATURES IN NORTHEAST ALASKA. SLIGHT PROBABILITIES FOR ABOVE-NORMAL TEMPERATURES REMAINED UNCHANGED FROM THE MID MONTH OUTLOOK IN THE SOUTHWEST DUE TO A COMBINATION OF FACTORS THAT INCLUDE FAVORED ENHANCED CLOUDINESS AND PRECIPITATION OFFSET BY STRONG LONG TERM TRENDS.
ACROSS THE EASTERN HALF OF THE CONUS, FAVORED WARMER THAN NORMAL CONDITIONS DURING THE SHORT-, MEDIUM- AND EXTENDED-RANGE PERIOD AND COMBINED TIME SCALE MONTHLY FORECAST TOOLS INDICATE A HIGH LIKELIHOOD FOR ABOVE NORMAL MONTHLY MEAN TEMPERATURES FOR SEPTEMBER. THE ODDS ARE GREATEST IN THE NORTHEAST AND LOWEST ALONG THE GULF COAST AND EXTREME SOUTHEAST AS A RESULT OF ELEVATED ODDS FOR ABOVE-NORMAL PRECIPITATION, ESPECIALLY EARLY IN THE MONTH.
AREAS SHOWN IN WHITE LABELED EC DEPICT A FORECAST WHERE THE PROBABILITIES FOR EITHER OF THE THREE CATEGORIES, BELOW-, NEAR- OR ABOVE-NORMAL MONTHLY MEAN TEMPERATURES ARE NOT SIGNIFICANTLY DIFFERENT FROM CLIMATOLOGICAL PROBABILITIES.
THE UPDATED SEPTEMBER PRECIPITATION OUTLOOK DEPICTS BOTH AREAS PERSISTING FROM THE MID MONTH OUTLOOK AS WELL AS AREAS REQUIRING SOME ADJUSTMENT FROM THE INITIAL OUTLOOK. FOR ALASKA, FORECAST GUIDANCE FAVORS POSITIVE MID-FLOW HEIGHT DEPARTURES DURING EARLY PARTS OF THE MONTH SO THE HIGHLIGHTED AREA FOR ABOVE NORMAL PRECIPITATION IS NOW ZONALLY ORIENTED ACROSS THE NORTHERN PORTION OF THE STATE RATHER THAN DEPICTED ALONG THE WESTERN COAST. IN ADDITION, BELOW-NORMAL PRECIPITATION IS NOW FAVORED FOR SOUTHERN AREAS OF THE STATE. FAVORED BELOW-NORMAL PRECIPITATION FROM THE EARLIER OUTLOOK FOR THE PACIFIC NORTHWEST REMAINS BUT IS SLIGHTLY EXPANDED AND ABOVE-NORMAL PRECIPITATION FAVORED FOR THE SOUTHWEST WAS SHIFTED EASTWARD BUT STILL INCLUDES MUCH OF THE SOUTHWEST AND CENTRAL AND SOUTHERN ROCKIES.
SHORT- AND MEDIUM-RANGE MODEL GUIDANCE INDICATES HIGH ODDS FOR AREAS OF HEAVY PRECIPITATION FOR AN REGION STRETCHING FROM NEW MEXICO TO THE GREAT LAKES DURING THE FIRST WEEK OF THE MONTH WITH THE GREATEST PROBABILITIES LOCATED IN THE UPPER MISSISSIPPI VALLEY. ABOVE-NORMAL PRECIPITATION REMAINS FAVORED ALONG THE EASTERN SEABOARD AS IN THE MID MONTH OUTLOOK, BUT THE AREA WAS EXTENDED ALONG THE GULF COAST AS A DISTURBANCE MOVING WEST-NORTHWESTWARD FROM THE WESTERN ATLANTIC IS FORECAST TO MOVE ALONG THE GULF COAST FROM FLORIDA TO TEXAS DURING THE FIRST WEEK OF THE MONTH.
AREAS SHOWN IN WHITE LABELED EC DEPICT A FORECAST WHERE THE PROBABILITIES FOR EITHER OF THREE CATEGORIES, BELOW-, NEAR- OR ABOVE-NORMAL MONTHLY TOTAL PRECIPITATION AMOUNTS ARE NOT SIGNIFICANTLY DIFFERENT FROM CLIMATOLOGICAL PROBABILITIES.
Week 3-4 Forecast Discussion Valid Sat Sep 15 2018-Fri Sep 28 2018
ENSO-neutral conditions continue across the equatorial Pacific Ocean where sea surface temperatures (SSTs) are near-to-above average. A transition towards El Nino is becoming more apparent with enhanced upper-level divergence (convergence) over the central Pacific (Indian Ocean). The Madden-Julian Oscillation (MJO) has remained weak during the past month. Dynamical models indicate enhanced convection over the Western Hemisphere with little to no eastward propagation. Therefore, the MJO is not expected to play a major role during the Week 3-4 period. The temperature and precipitation outlooks are based on the evolving circulation pattern from Week-2, dynamical model forecasts from the NCEP CFS, ECMWF, and JMA, and a multi-model ensemble (MME) of experimental and operational ensemble prediction systems. Long-term trends are also considered.
Positive 500-hpa height anomalies are forecast across much of the mid to higher latitudes of North America during Week-2. The CFS and ECMWF models maintain this broad 500-hpa height pattern through Week-3 with the largest positive anomalies over northwest North America. These dynamical models diverge during Week-4. The ECMWF model indicates an upper-level trough shifting east to the Pacific Northwest, while the CFS model features a persistence of positive 500-hpa height anomalies in this region. Despite these diverging model solutions during Week-4, the temperature tools are in reasonably good agreement and feature increased chances for above-normal temperatures across the western and north-central U.S. The highest odds (above 70%) for above normal temperatures are forecast for parts of the southwestern U.S. based on long-term trends. More uncertainty exists in the temperature outlook across the eastern U.S. where the MME indicates weak signals. Above-normal temperatures are more likely across western Alaska, the Aleutians, and the Alaska Panhandle based on dynamical model guidance and above-normal SSTs along western mainland Alaska. Temperature tools generally agree on below-normal temperatures for northeast Alaska.
The East Pacific is likely to remain active with a number of tropical cyclones forming during September, which elevates the chances of a moisture surge north into the Desert Southwest. Also, the 500-hpa height anomaly pattern implies easterly waves crossing the lower latitudes. Based on these two factors along with model guidance, above-median precipitation is favored across the desert Southwest and southern to central Rockies. A predicted upper-level ridge over western North America, at least through Week-3, increases chances for below-median precipitation from the Pacific Northwest east to the northern high Plains. Despite the recent wet pattern across the Great Lakes, most precipitation tools have their strongest dry signal across this region during Weeks 3-4. This favored area for below-median precipitation extends east to include the Northeast. The ECMWF and JMA models indicate enhanced odds for above median precipitation across the Southeast, especially along the Gulf and Atlantic coasts. This is consistent with the expectation that enhanced upper-level divergence continues across the Western Hemisphere during the next few weeks.
Sea surface temperatures remain slightly above normal surrounding Hawaii, and the consensus of dynamical model guidance, including the experimental SubX models, indicates enhanced probabilities for above-normal temperatures. Dynamical model precipitation forecasts favor above-median rainfall throughout the Hawaiian Islands.
Visual Consistency Testing.
Sometimes it is useful to see how the Monthly forecast fits with the 1 – 5 Day, 6 – 10 Day , 8 – 14 Day and Week 3 and 4 forecasts. This approach actually works better for the end of month update than for the Early Outlook for the subsequent month as we have forecasts for pars of the month that pretty much cover the full month. I do not have comparable maps for the Day 1 – 5 forecast in the same format as the three maps we generally work with. What I am showing for temperature is the Day 3 Maximum Temperature and for precipitation the five-day precipitation: the latter being fairly similar in format to the subsequent set of the maps I present each week but showing absolute QPF (inches of precipitation) not QPF deviation from Normal.
First Temperature
And then Precipitation
Sometimes it is useful to compare the forecast for the current month with the prior three-month forecast to see if the updated forecast for September might impact the prior forecast for September/October/November issued two weeks ago.
September, 2018 Plus September – November 2018 Outlook
One can mentally subtract the First-Month Outlook from the Three-month Outlook and create the Outlook for the last two months in the three-month period.
*The concept is that the probabilities of a deviation from climatology in the First Month and the combined Month Two and Three forecast that one derives must average out to the probabilities shown in the three-month maps.
For comparison purposes, here are the NOAA September – October – November forecast maps on the first set of rows and the corresponding JAMSTEC forecast maps for North America in the second set of rows.
How does this impact the drought outlook?
You can access our August 30, 2018 Drought and Agriculture Report by clicking on:
August 30, 2018 Drought and Agriculture Report – Drought Persists
Written by Sig Silber