Written by Sig Silber
Updated at 11 PM EDT April 2, 2021 to reflect the Week 3-4 Forecast that was issued today. In some ways, it supports the full month forecast and discussion issued on March 31, 2021, and in other ways, it disagrees with it. The way to look at this is that the signals are not strong and some of the models disagree so on any given day the overall picture looks a bit different. Both in this article and in our LIVE Article which we update at least twice a day, there is the same set of partial forecasts as here but without our commentary but there is updated commentary from NOAA. So for those who have an interest in the latest information, it can be found in our LIVE article on the econintersect.com website. The “Perhaps” in our title was wise as it still appears to be a bit less dry but not quite as much as earlier. But it does now appear to be a bit less warm.
At the end of every month, NOAA issues an update for the following month to their Early Outlook, issued as usual on the Third Thursday of March. Today, thirteen days later, NOAA has issued their Updated Outlook for April. We are seeing the first baby steps in transitioning from La Nina to ENSO Neutral. The changes from the Early Outlook forecast issued on March 18, 2021 are not huge. It is a bit less dry but it is by no means a major change. There is some difference of opinion between the weather forecast and the drought forecast. It is subtle and small improvements in precipitation do not immediately translate into upgrades in drought status where there has been prolonged D3 and D4 Drought.
Some housekeeping: On March 18, 2021, we published Part I of our analysis of the NOAA and JAMSTEC Three -to Four -Season Outlook and that can be accessed here. In that report, the April 2021 Early Outlook was issued. This article presents the NOAA update of their Early temperature and precipitation Outlooks for April and the most recent Drought Forecast update for the month of April.
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This was not part of today’s NOAA Update.
First, we will compare the NOAA Early Outlook for April 2021 with the newly issued update.
Early Outlook Temperature
Updated Temperature Forecast
Early Outlook Precipitation
Updated Precipitation Forecast
Here is the discussion released with the forecast.
30-DAY OUTLOOK DISCUSSION FOR APRIL 2021
The updated April 2020 outlook is based on the April outlook released in mid-March adjusted using the current WPC outlooks for week 1 and CPC outlooks for the extended range and week 3-4, while considering current climate conditions, including current La Nina conditions, and following the most recent subseasonal and monthly integrated dynamical model guidance. Although La Nina conditions persist over the tropical Pacific, the weekly Nino 3.4 sea surface temperature (SST) anomaly was observed to be about -0.5 degree C, and conditions are likely to transition to ENSO-neutral during spring. The Madden Julian Oscillation (MJO) active phase is currently over the Indian Ocean as determined by the RMM indices, and forecast by dynamical models to progress into the Western Pacific in the next couple weeks. The impacts of the current MJO and ENSO state and the decadal trends are mostly consistent with current dynamical model forecasts for temperature and precipitation for April.
The updated April temperature outlook has only small changes relative to the prior outlook. Most notably, the area of likely below normal temperatures in the Pacific Northwest increased, while the area of probable above normal temperatures over the West decreased. Probabilities of above normal temperatures over parts of the Northern Plains, western Great Lakes region, and northern New England increased to be over 60 percent. Dynamical models predict ridging generally over the CONUS during April, with some eastward progression from the western to the eastern CONUS during the month. While above normal temperatures are expected for most of the West, except for near coastal areas, early in the month (based on WPC outlooks), near to below normal temperatures are favored from the Pacific coast to the Rocky Mountains in the recent CPC 6-10 day and week 2 outlooks, leading to an increased probability for below normal temperatures for much of the Pacific Northwest in the updated April outlook, and equal chances (EC) of above, near and below normal temperatures for much of the remaining areas west of the Rockies. Enhanced probabilities of above normal temperatures remain for interior parts of Southern California, eastern Nevada, the Northern Rockies, and the rest of the CONUS to the east coast, in the updated April outlook. Confident outlooks for above normal temperatures for eastern areas of the Northern Plains and western areas of the Great Lakes region, throughout the month of April, and for northern areas in the Northeast from the 6-10 day to the week 3-4 periods, led to an increase in the probabilities of above normal temperatures in the updated April outlook. Expected below normal temperatures early in the month of April for much of the Southeast, offset by likely above normal temperatures in the CPC 6-10 day, week 2, and week 3-4 outlooks, led to a decrease in the probabilities of above normal temperatures for this region in the monthly update. Below normal temperatures are favored in both the half-month lead and updated monthly outlook for southern Mainland Alaska and the Alaska Panhandle, with a persistent circulation pattern and troughing over the state. Above normal temperatures continue to be favored for northern areas of the state of Alaska, supported by decadal temperature trends .
The updated April precipitation outlook indicates some persistence of the relatively dry pattern forecast over the CONUS at a half-month lead, with some notable changes. The WPC forecasts precipitation for Northern California and western Oregon in the first few days of April, which are near climatological normal amounts for these areas. The CPC 6-10 day outlook calls for near normal precipitation, and the week 3-4 outlook indicates equal chances of above and below median precipitation for much of the West. These outlooks are consistent with recent CFS precipitation forecasts for April that predict near to above normal precipitation for much of the West. Therefore, the chances of below normal precipitation have decreased relative to the half-month lead April outlook for southern Oregon, western Idaho, Northern California, and western Nevada, and equal chances of above, near and below normal precipitation are indicated in the monthly update. Uncertainty has increased and probabilities of below normal precipitation have decreased for west Texas and parts of the eastern Southern Plains states, due to weak probabilities for above normal precipitation in the extended range. Probabilities for above normal precipitation have increased for parts of the Northern Plains into the far western Great Lakes region, as outlooks across all lead times during April predict slightly enhanced probabilities of above normal precipitation. The area of likely above normal precipitation for western areas of Mainland Alaska have expanded into parts of the Alaska Interior, with persistent enhanced westerly flow in model forecasts.
March 18, 2021 Forecast for April 2021 | March 31, 2021, Forecast for April 2021 | |
Temperature | ||
Precipitation |
Visual Consistency Testing.
It is useful to see how the full-month forecast fits with the set of partial-month forecasts that we have. These will continue to update in this article. It is important to remember that with the partial-month forecasts, we generally have about 25 days of the subsequent month to look at when we do this at the end of the month as we are doing now. It depends on what day of the week the month ends. So this month, which ended on a Wednesday, we have short-term forecasts that cover 23 of the 30 days in April so we are in fair shape to see if the whole is equal to the sum of the parts. As of this update we now have short-term forecasts for the remaining 28 days of April so we are in a perfect position to assess the consistency in the forecast for the month.
First Temperature
And Precipitation
Here is the discussion that was released with the Week 3 – 4 Forecast on April 2, 2021
Week 3-4 Forecast Discussion Valid Sat Apr 17 2021-Fri Apr 30 2021
In the tropical Pacific, La Nina conditions persist but the anomalously cool sea surface temperature (SST) anomalies have slowly weakened over the last several weeks and the most recent ENSO forecast slightly favors a transition to neutral conditions later this spring. The Madden Julian Oscillation (MJO) is currently active over the Maritime Continent. Dynamical forecast guidance consistently progresses this signal eastward over the West Pacific throughout the next two weeks. As we progress into the spring and summer months, these tropical phenomena will have less of an influence on the US sensible weather. Both the Arctic Oscillation and the North Atlantic Oscillation are in the positive phase and are currently forecast to be neutral/slightly negative by the start of the forecast period. As such, this week’s outlook is largely based on a blend of the dynamical model guidance, with additional considerations from decadal trends.
The agreement among the dynamical models (CFS, ECMWF, JMA, and the SubX multi-model ensemble) is modest regarding the anomalous 500-hPa pattern over the Week 3-4 period. The models consistently forecast ridging centered near the Aleutian Islands over the North Pacific that extends over much of Alaska. The models vary more on the predicted height patterns over the CONUS with the CFS indicating a broad region of positive heights and the JMA favoring weakly negative heights across the northern tier. The ECMWF suggests an intermediate solution with ridging on the West Coast and weak troughing over the East.
As a result of the uncertainty in the forecast height patterns, equal chances of anomalous temperatures are carved out for much of the eastern and western portions of the CONUS. The temperature forecast tilts toward above normal probabilities through the Southwest and the Central Rockies where long term trends are positive and the dynamical models collectively agree on anomalous ridging. The highest confidence of above normal temperatures is predicted across southern New Mexico and western Texas where probabilities exceed 60 percent. Above normal temperatures are also slightly favored along the Gulf Coast and Florida, consistent with the surrounding positive SST anomalies. Dynamical model guidance favors below normal temperature probabilities over southwest Alaska due to predicted northerly flow while above normal temperature chances increase along the northern regions of the state.
The precipitation forecast favors dry conditions over the Pacific Northwest and along the panhandle and southern coast of Alaska while odds grow for wet conditions in the northern portion of the state.The dynamical models and the SubX ensemble consistently indicate a broad region of below median probabilities across the East Coast extending across the South and through the Central Rockies under the anticipated weak ridging. Elevated probabilities of above median precipitation are forecast over the Northern Plains.
Near normal SSTs are observed surrounding Hawaii and dynamical model guidance suggests equal chances for much of the state with a slight tilt to above normal temperatures over the northwest islands. Dynamical model guidance also indicates equal chances of anomalous precipitation across the region.
Sometimes it is useful to compare the three-month outlook to the forecast for the first of the three months. It shows how much the pattern changes over the three-month period.
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.
And with respect to drought, this was also issued on March 31, 2021
Latest Monthly Assessment – A La Niña Advisory remains in effect, with a 60% chance that La Niña conditions will transition to ENSO-neutral during the Northern Hemisphere Spring. As such, La Niña will likely influence conditions across the U.S. during April.
Drought conditions have persisted across the western CONUS during March, with only minor improvements observed following the passages of transient storm systems. Parts of the Pacific Northwest have received above-normal seasonal precipitation since the fall 2020, which has resulted in above-normal snowpack and reservoir levels leading up to April. However, March precipitation remained below-normal for most locations. With below-normal temperatures and equal chances of above or below-normal precipitation in the Pacific Northwest, and a lack of temperature and precipitation signals along the central and southern West Coast, drought persistence is likely throughout the West. Only D1 areas along the windward slopes of the Cascades in Oregon might see some drought removal. The Central Plains experienced a strong storm system that dropped heavy snow in parts of southeastern Wyoming and eastern Colorado, and heavy rain eastward to the Midwest, in mid-March. Meanwhile, the Dakotas saw continued drought intensification from below-normal seasonal snowpack and above-normal temperatures during March. Wet signals across the Northern Plains during April are not strong enough to indicate major improvements are likely for locations entrenched in severe (D2) and extreme (D3) drought. However, removal may occur for some locations in the eastern Dakotas, as antecedent dryness is not as pronounced as areas farther west. Drought persistence, with some additional development in abnormally dry (D0) areas, is also likely across the Central and Southern Plains. Despite varying wet signals, temperatures are likely to remain above-normal throughout April. The Plains are also prone to high-wind events during the period, which could exacerbate dry conditions further. The upper Midwest, Tennessee Valley, middle Mississippi Valley, and the southern Louisiana coast received beneficial rainfall during March, warranting some improvements in drought conditions, and staving off further development in these regions during April, given the weakly below-normal to lack of a precipitation signal. Locations in the central and eastern Great Lakes and Northeast have missed out on some of the heavier precipitation from passing storm systems, increasing coverage of abnormally dry (D0) areas and leading to some expansion of moderate (D1) drought. Above-normal temperatures are favored across the region, and with the lack of a precipitation signal (leaning below-normal toward the Northeast in some of the forecast tools), conditions are likely to deteriorate. Conditions in the Northeast may also be exacerbated by an early start to vegetation growth, due to increased water uptake from the soils and increased evapotranspiration rates. In Florida, antecedent dryness, increased odds of above-normal temperatures and below-normal precipitation, and April typically being a dry month climatologically, indicate a greater likelihood for deterioration across the Florida Peninsula.
Looking back on March 2021 to relate the forecast for April to the actuals in March.
First March 2021 Temperature (30 out of 31 days).
And then March Precipitation (30 out of 31 days).
We then show the new forecast and the prior month’s actuals (less one day) side by side.
Prior Month (usually missing one day) | Forecast for current Month | |
Temperature | ||
Precipitation |
Conclusion
The purpose of this article was to present the updated Outlook for April 2021 and compare it with the Early Outlook. It is not to critique the updated Outlook for April based on our opinions but we point out possible inconsistencies if we find them and we have. We have assessed the extent to which the monthly outlook seems to fit with the other forecasts provided by NOAA. In reality, all forecasts are obsolete when issued as the situation changes day by day or six hours by six hours i.e. each new model run. In our other articles, we continually examine the current forecasts from NOAA including Day 1- 5, 6-10, 8-14, and Week 3 and 4. So we are continually looking at approximately 25 days into the future.
On April 8, 2021, IRI/CBC will issue their ENSO forecast and we will present that and critique it. What they present on April 8 is likely to begin to impact the forecasts for the next three months and should shed some light on this Summer. It is an ongoing process and we attempt to provide our readers with the best current assessment of the next approximately 25 days (except when we cover the Four-Season Analysis or medium-frequency cycles such as ENSO in which cases we are looking further out than 25 days), so our analysis is pretty much ongoing. We also publish a “LIVE” Weather Article which updates in real-time to reflect NOAA Weather Headlines and for those interested provides detailed information down to current warnings in place. All of our reports including past versions of these reports can be located via the Directory/Archive which can be accessed here.