stated in 14 CFR part 91, section 91.3 should an immediate  deviation from the assigned clearance be necessary and time  does not permit approval by ATC. Generally, when weather disrupts the flow of air traffic,  greater workload demands are placed on the controller.  Requests for deviations from course and other services  should be made as far in advance as possible to better  assure the controller’s ability to approve these requests  promptly. When requesting approval to detour around  weather activity, include the following information to  facilitate the request: To a large degree, the assistance that might be rendered  by ATC depends upon the weather information available  to controllers. Due to the extremely transitory nature of  hazardous weather, the controller’s displayed precipitation  information may be of limited value. Obtaining IFR clearance or approval to circumnavigate  hazardous weather can often be accommodated more readily  in the en route areas away from terminals because there  is usually less congestion and, therefore, greater freedom  of action. In terminal areas, the problem is more acute  because of traffic density, ATC coordination requirements,  complex departure and arrival routes, and adjacent airports.  As a consequence, controllers are less likely to be able to  accommodate all requests for weather detours in a terminal  area. Nevertheless, pilots should not hesitate to advise  controllers of any observed hazardous weather and should  specifically advise controllers if they desire circumnavigation  of observed weather. Pilot reports (PIREPs) of flight conditions help define the  nature and extent of weather conditions in a particular area.  These reports are disseminated by radio and electronic means  to other pilots. Provide PIREP information to ATC regarding  pertinent flight conditions, such as: The PRM uses a Monopulse Secondary Surveillance Radar  (MSSR) that employs electronically-scanned antennas.  Because the PRM has no scan rate restrictions, it is capable  of providing a faster update rate (up to 1.0 second) over  conventional systems, thereby providing better target  presentation in terms of accuracy, resolution, and track  prediction. The system is designed to search, track, process,  and display SSR-equipped aircraft within airspace of over  30 miles in range and over 15,000 feet in elevation. Visual  and audible alerts are generated to warn controllers to take  corrective actions.  Over the past few years, a new technology has been installed  at airports that permits a decreased separation distance  between parallel runways. The system is called a Precision  Runway Monitor (PRM) and is comprised of high-update  radar, high-resolution ATC displays, and PRM-certified  controllers.  [Figure 2-14] An approach control facility is a terminal ATC facility  that provides approach control service in the terminal area.  Services are provided for arriving and departing VFR and  IFR aircraft and, on occasion, en route aircraft. In addition,  for airports with parallel runways with ILS or LDA  approaches, the approach control facility provides monitoring  of the approaches.  Approach Control Facility Approach Control Advances Precision Runway Monitor (PRM) 1.  The proposed point where detour commences;  2.  The proposed route and extent of detour (direction  and distance); 3.  The point where original route will be resumed; 4.  Flight conditions (instrument meteorological  conditions (IMC) or visual meteorological conditions  (VMC); 5.  Whether the aircraft is equipped with functioning  airborne radar; and 6.  Any further deviation that may become necessary. 3.  Cloud tops and bases; and 4.  The presence of hazards such as ice, hail, and lightning. 1.  Turbulence; 2.  Visibility; Figure 2-14.  High-resolution ATC displays used in PRM. PRM Radar 2-12